feat: add Japanese localization with String Catalog (#819)

* Add i18n infrastructure with String Catalog and Japanese translations

Introduce String Catalog (.xcstrings) for localization support:
- Localizable.xcstrings: 195 UI string entries with en and ja translations
- InfoPlist.xcstrings: Info.plist strings (microphone usage, Finder menu items)
- project.pbxproj: add xcstrings to build phase and ja to knownRegions

* Replace hardcoded UI strings with String(localized:defaultValue:)

Migrate all user-facing strings across 11 source files to use
String(localized:defaultValue:) API (macOS 13+). Each string references
a key in Localizable.xcstrings, with the English text preserved as
defaultValue for fallback.

Files modified:
- KeyboardShortcutSettings: 28 shortcut labels
- SocketControlSettings: mode names and descriptions
- TabManager: placement labels, color names, close dialogs
- BrowserPanel/BrowserPanelView: error pages, context menus, tooltips
- UpdateViewModel/UpdatePopoverView/UpdatePill: update UI states
- NotificationsPage: notification panel labels
- SurfaceSearchOverlay: search bar placeholder and tooltips
- AppDelegate: menus, dialogs, command palette items

* Fix localization gaps from review feedback

Address review comments from CodeRabbit, Greptile, and Cubic Dev AI:
- Use interpolated String(localized:) instead of concatenation for
  version/progress strings in UpdateViewModel
- Localize remaining hardcoded strings in AppDelegate: window labels,
  rename dialog, status menu items, unread notification count
- Localize insecure HTTP alert body in BrowserPanel
- Add 12 new entries to Localizable.xcstrings with Japanese translations

* Fix String(localized:defaultValue:) keys to use StaticString

The localized: parameter requires StaticString when defaultValue: is
used. Move string interpolation from the key to defaultValue only,
and revert maxWidthText to plain strings since they are only used for
layout width calculation.

* Localize remaining UI strings across all source files

Add String(localized:defaultValue:) to all user-facing strings in:
- cmuxApp.swift: settings screen, menus, about panel, dialogs (~180 strings)
- ContentView.swift: command palette, sidebar context menu, dialogs (~200 strings)
- Workspace.swift: rename/move/close tab dialogs, tooltips (~20 strings)
- UpdateTitlebarAccessory.swift: titlebar tooltips, notifications popover (~10 strings)
- TerminalNotificationStore.swift: notification permission dialog (4 strings)
- CmuxWebView.swift: browser context menu items (2 strings)
- AppDelegate.swift: CLI install/uninstall alerts (6 strings)

Add 418 new entries to Localizable.xcstrings with Japanese translations.
Extract sidebar context menu into separate @ViewBuilder to fix Swift
type-checker timeout in large body.
Fix xcstrings format specifiers for interpolated strings (%lld, %@).

Total: 624 localization entries covering the full UI.

* Address review feedback: fix missing localizations and terminology

- Localize javaScriptDialogTitle URL branch in BrowserPanel
- Localize cantReach error message in BrowserPanel
- Localize close other tabs dialog message in TabManager
- Localize workspace accessibility label in ContentView
- Fix unread notification singular/plural (split into two keys)
- Fix insecure connection apostrophe inconsistency (unify to U+2019)
- Rename socketControl.fullOpen.description to socketControl.allowAll.description
- Remove dead code: renameTargetNoun function
- Fix terminology inconsistencies in xcstrings:
  - Unify "Developer Tools" to デベロッパツール
  - Unify "Jump to Latest Unread" phrasing
  - Unify "Flash Focused Panel" terminology
  - Fix dialog.enableNotifications.notNow translation

* fix: address remaining PR 819 review feedback

* fix: use a single localized key for close-other-tabs

* fix: avoid inflection markup in close-other-tabs message

* Address review feedback: localize tooltip, fix subtitle concat, unify keys

- Localize menubar tooltip unread count (hardcoded English -> localized)
- Replace subtitle string concatenation anti-pattern with single localized
  keys containing interpolation placeholders
- Unify workspace fallback key to workspace.displayName.fallback
- Remove unused workspace.defaultName key from xcstrings
- Add Japanese translations for new tooltip and subtitle keys

Sources/Update/UpdateTitlebarAccessory.swift

@@ -320,8 +320,8 @@ struct TitlebarControlsView: View {
                 iconLabel(systemName: "sidebar.left", config: config)
             }
             .accessibilityIdentifier("titlebarControl.toggleSidebar")
-            .accessibilityLabel("Toggle Sidebar")
-            .help(KeyboardShortcutSettings.Action.toggleSidebar.tooltip("Show or hide the sidebar"))
+            .accessibilityLabel(String(localized: "titlebar.sidebar.accessibilityLabel", defaultValue: "Toggle Sidebar"))
+            .help(KeyboardShortcutSettings.Action.toggleSidebar.tooltip(String(localized: "titlebar.sidebar.tooltip", defaultValue: "Show or hide the sidebar")))
 
             TitlebarControlButton(config: config, action: {
                 #if DEBUG
@@ -347,8 +347,8 @@ struct TitlebarControlsView: View {
             }
             .accessibilityIdentifier("titlebarControl.showNotifications")
             .background(NotificationsAnchorView { viewModel.notificationsAnchorView = $0 })
-            .accessibilityLabel("Notifications")
-            .help(KeyboardShortcutSettings.Action.showNotifications.tooltip("Show notifications"))
+            .accessibilityLabel(String(localized: "titlebar.notifications.accessibilityLabel", defaultValue: "Notifications"))
+            .help(KeyboardShortcutSettings.Action.showNotifications.tooltip(String(localized: "titlebar.notifications.tooltip", defaultValue: "Show notifications")))
 
             TitlebarControlButton(config: config, action: {
                 #if DEBUG
@@ -359,8 +359,8 @@ struct TitlebarControlsView: View {
                 iconLabel(systemName: "plus", config: config)
             }
             .accessibilityIdentifier("titlebarControl.newTab")
-            .accessibilityLabel("New Workspace")
-            .help(KeyboardShortcutSettings.Action.newTab.tooltip("New workspace"))
+            .accessibilityLabel(String(localized: "titlebar.newWorkspace.accessibilityLabel", defaultValue: "New Workspace"))
+            .help(KeyboardShortcutSettings.Action.newTab.tooltip(String(localized: "titlebar.newWorkspace.tooltip", defaultValue: "New workspace")))
         }
 
         let paddedContent = content.padding(config.groupPadding)
@@ -901,11 +901,11 @@ private struct NotificationsPopoverView: View {
     var body: some View {
         VStack(spacing: 0) {
             HStack {
-                Text("Notifications")
+                Text(String(localized: "notifications.title", defaultValue: "Notifications"))
                     .font(.headline)
                 Spacer()
                 if !notificationStore.notifications.isEmpty {
-                    Button("Clear All") {
+                    Button(String(localized: "notifications.clearAll", defaultValue: "Clear All")) {
                         notificationStore.clearAll()
                     }
                     .buttonStyle(.bordered)
@@ -921,9 +921,9 @@ private struct NotificationsPopoverView: View {
                     Image(systemName: "bell.slash")
                         .font(.system(size: 28))
                         .foregroundColor(.secondary)
-                    Text("No notifications yet")
+                    Text(String(localized: "notifications.empty.title", defaultValue: "No notifications yet"))
                         .font(.headline)
-                    Text("Desktop notifications will appear here.")
+                    Text(String(localized: "notifications.empty.subtitle", defaultValue: "Desktop notifications will appear here."))
                         .font(.subheadline)
                         .foregroundColor(.secondary)
                 }