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/maintenance/BatchLaunchModal.tsx

@@ -76,7 +76,7 @@ export function BatchLaunchModal({ treeId, treeName, onClose, onLaunched }: Batc
   ]
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-sm">
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/60 backdrop-blur-xs">
       <div className="w-full max-w-lg rounded-xl border border-border bg-card shadow-2xl">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border px-6 py-4">
@@ -116,7 +116,7 @@ export function BatchLaunchModal({ treeId, treeName, onClose, onLaunched }: Batc
                 Server names (one per line)
               </label>
               <textarea
-                className="h-40 w-full rounded-lg border border-border bg-card px-3 py-2 text-[0.875rem] text-foreground placeholder:text-muted-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+                className="h-40 w-full rounded-lg border border-border bg-card px-3 py-2 text-[0.875rem] text-foreground placeholder:text-muted-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
                 placeholder={"RDS-01\nRDS-02\nRDS-03"}
                 value={manualInput}
                 onChange={e => setManualInput(e.target.value)}

frontend/src/components/maintenance/BatchStatusCard.tsx

@@ -132,7 +132,7 @@ export function BatchStatusCard({ session, targetLabel, treeId, batchId }: Batch
         {isNotStarted && (
           <button
             onClick={handleStart}
-            className="flex items-center gap-1.5 rounded-lg bg-gradient-brand px-3 py-1.5 text-[0.8125rem] font-medium text-white shadow-sm shadow-primary/20 hover:opacity-90 transition-opacity"
+            className="flex items-center gap-1.5 rounded-lg bg-gradient-brand px-3 py-1.5 text-[0.8125rem] font-medium text-white shadow-xs shadow-primary/20 hover:opacity-90 transition-opacity"
           >
             <Play className="h-3.5 w-3.5" />
             Start