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>

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

frontend/src/pages/TreeEditorPage.tsx

@@ -528,7 +528,7 @@ export function TreeEditorPage() {
 
       {/* Draft Restore Prompt */}
       {showDraftPrompt && (
-        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-xs">
           <div className="bg-card border border-border rounded-xl w-full max-w-md p-6 shadow-lg">
             <h2 className="mb-2 text-lg font-heading font-semibold text-foreground">Restore Draft?</h2>
             <p className="mb-4 text-sm text-muted-foreground">
@@ -560,7 +560,7 @@ export function TreeEditorPage() {
 
       {/* Unsaved Changes Dialog */}
       {blocker.state === 'blocked' && (
-        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-xs">
           <div className="bg-card border border-border rounded-xl w-full max-w-md p-6 shadow-lg">
             <h2 className="mb-2 text-lg font-heading font-semibold text-foreground">Unsaved Changes</h2>
             <p className="mb-4 text-sm text-muted-foreground">