feat(ui): UI/UX polish — layout, sidebar, button, theme improvements

- Fix global scrollbar overflow by removing h-svh from html element
- Add h-full overflow-hidden to html/body for proper app-like layout
- Fix default button variant: add shadow-sm and hover:bg-primary/90
- Update sidebar create-issue button to bg-background with shadow
- Add WorkspaceAvatar component and search/new-issue actions to sidebar header
- Improve theme provider with TooltipProvider wrapper
- Polish various page layouts, pickers, modals, and code block styling
- Clean up custom.css unused styles

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/app/custom.css

@@ -25,7 +25,8 @@
   animation: entrance-spin 0.6s ease-out forwards;
 }
 
-/* Shadcn sidebar: remove default padding from inset container */
-[data-slot="sidebar-container"] {
-  padding: 0 !important;
+/* Sidebar: open triggers (dropdown/popover) get active background */
+[data-sidebar="menu-button"][data-popup-open] {
+  background-color: var(--sidebar-accent);
+  color: var(--sidebar-accent-foreground);
 }