chore: Tailwind CSS v3 → v4 migration (#99)

* chore: run Tailwind v4 upgrade tool (Phase 1)

- Upgraded tailwindcss v3 → v4.2.1, postcss plugin to @tailwindcss/postcss
- Deleted tailwind.config.js, migrated theme to CSS @theme block in index.css
- Replaced @tailwind directives with @import 'tailwindcss'
- Added @custom-variant dark, @utility blocks for custom utilities
- Updated class names across 128 files (shadow-sm → shadow-xs, etc.)
- Removed autoprefixer (built into v4)
- Added migration plan doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: switch from @tailwindcss/postcss to @tailwindcss/vite (Phase 2)

- Replaced @tailwindcss/postcss with @tailwindcss/vite plugin
- Deleted postcss.config.js (no longer needed)
- Tailwind now runs as a native Vite plugin for faster HMR

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: convert to OKLCH colors, move keyframes into @theme (Phase 3-4)

- Replaced all HSL color indirection with direct OKLCH values in @theme
- Moved all keyframes inside @theme block (v4 pattern)
- Eliminated hsl(var(--x)) double-indirection across 17 component files
- Replaced hsl() inline styles with var(--color-*) theme references
- Cleaned up redundant rdp-* utility blocks
- Fixed @custom-variant dark syntax to use :where()
- Added sidebar/glass/shadow vars as OKLCH in :root

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/ContinuationModal.tsx

@@ -62,7 +62,7 @@ export function ContinuationModal({
                     'hover:border-border hover:bg-accent'
                   )}
                 >
-                  <div className="flex h-8 w-8 flex-shrink-0 items-center justify-center rounded-full bg-accent">
+                  <div className="flex h-8 w-8 shrink-0 items-center justify-center rounded-full bg-accent">
                     {nodeTypeIcons[node.type]}
                   </div>
                   <div className="min-w-0 flex-1">
@@ -71,7 +71,7 @@ export function ContinuationModal({
                       {nodeTypeLabels[node.type]}
                     </p>
                   </div>
-                  <ArrowRight className="h-4 w-4 flex-shrink-0 text-muted-foreground" />
+                  <ArrowRight className="h-4 w-4 shrink-0 text-muted-foreground" />
                 </button>
               ))}
             </div>
@@ -98,7 +98,7 @@ export function ContinuationModal({
               'hover:border-amber-500 hover:bg-amber-500/10'
             )}
           >
-            <div className="flex h-10 w-10 flex-shrink-0 items-center justify-center rounded-full bg-amber-500/10">
+            <div className="flex h-10 w-10 shrink-0 items-center justify-center rounded-full bg-amber-500/10">
               <GitBranch className="h-5 w-5 text-amber-500" />
             </div>
             <div className="flex-1">
@@ -111,7 +111,7 @@ export function ContinuationModal({
 
           {/* Warning */}
           <div className="mt-3 flex items-start gap-2 rounded-md bg-yellow-400/10 p-3">
-            <AlertTriangle className="mt-0.5 h-4 w-4 flex-shrink-0 text-yellow-400" />
+            <AlertTriangle className="mt-0.5 h-4 w-4 shrink-0 text-yellow-400" />
             <p className="text-sm text-yellow-400">
               You'll need to complete this branch manually or mark the issue as resolved.
               Custom branches can be saved as a personal tree when your session ends.