Add Match Terminal Background sidebar setting (#2293)

* Add "Match Terminal Background" sidebar setting

Adds a toggle in Settings > Sidebar Appearance that makes the sidebar
use the same background color and transparency as the terminal area.

Uses layer-level opacity on a fully opaque background color (the same
technique as TitlebarLayerBackground) with effective opacity formula
`1 - (1-alpha)^2` to account for the terminal's two stacked
semi-transparent layers (Bonsplit chrome + Ghostty Metal surface).

Also adds a 1px trailing border derived from the terminal chrome color,
matching the bonsplit tab bar separator logic.

* Fix sidebar border color not updating on theme change

Add @State + .onReceive(.ghosttyDefaultBackgroundDidChange) to
SidebarTrailingBorder so the separator color recomputes when the
Ghostty theme changes, matching the pattern used in SidebarBackdrop.

* Address review comments: localize debug toggle, fix separator refresh

- Localize the debug panel toggle label (Codex P1)
- Add .onAppear to SidebarTrailingBorder for initial color (Cubic P2)
- Fix stale doc comment on SidebarTerminalBackgroundView (Cubic P3)

---------

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>

Resources/Localizable.xcstrings

@@ -83198,6 +83198,40 @@
         }
       }
     },
+    "settings.sidebarAppearance.matchTerminalBackground": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Match Terminal Background"
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "ターミナルの背景に合わせる"
+          }
+        }
+      }
+    },
+    "settings.sidebarAppearance.matchTerminalBackground.subtitle": {
+      "extractionState": "manual",
+      "localizations": {
+        "en": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "Use the same background color and transparency as the terminal."
+          }
+        },
+        "ja": {
+          "stringUnit": {
+            "state": "translated",
+            "value": "ターミナルと同じ背景色と透明度を使用します。"
+          }
+        }
+      }
+    },
     "settings.sidebarAppearance.tintColorLight": {
       "extractionState": "manual",
       "localizations": {