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/procedural-editor/MaintenanceScheduleSection.tsx

@@ -154,7 +154,7 @@ export function MaintenanceScheduleSection({ treeId, onScheduleLoaded }: Mainten
         <select
           value={frequency}
           onChange={(e) => setFrequency(e.target.value)}
-          className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+          className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
         >
           {FREQUENCY_OPTIONS.map(opt => (
             <option key={opt.value} value={opt.value}>{opt.label}</option>
@@ -172,7 +172,7 @@ export function MaintenanceScheduleSection({ treeId, onScheduleLoaded }: Mainten
             max={23}
             value={hour}
             onChange={(e) => setHour(Number(e.target.value))}
-            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
           />
         </div>
         <div>
@@ -183,7 +183,7 @@ export function MaintenanceScheduleSection({ treeId, onScheduleLoaded }: Mainten
             max={59}
             value={minute}
             onChange={(e) => setMinute(Number(e.target.value))}
-            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
           />
         </div>
       </div>
@@ -194,7 +194,7 @@ export function MaintenanceScheduleSection({ treeId, onScheduleLoaded }: Mainten
         <select
           value={timezone}
           onChange={(e) => setTimezone(e.target.value)}
-          className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+          className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
         >
           {TIMEZONE_OPTIONS.map(tz => (
             <option key={tz} value={tz}>{tz}</option>
@@ -209,7 +209,7 @@ export function MaintenanceScheduleSection({ treeId, onScheduleLoaded }: Mainten
           <select
             value={selectedTargetListId}
             onChange={(e) => setSelectedTargetListId(e.target.value)}
-            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20"
+            className="w-full rounded-lg border border-border bg-card px-3 py-2 text-foreground focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20"
           >
             <option value="">None — manual targets only</option>
             {targetLists.map(tl => (