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/step-library/StepForm.tsx

@@ -179,7 +179,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
           onChange={(e) => setTitle(e.target.value)}
           placeholder="Enter step title"
           className={cn(
-            'w-full rounded-md border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20',
+            'w-full rounded-md border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20',
             errors.title ? 'border-red-400/50' : 'border-border'
           )}
         />
@@ -201,7 +201,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
           placeholder="Describe what to do in this step..."
           rows={6}
           className={cn(
-            'w-full rounded-md border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20',
+            'w-full rounded-md border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20',
             errors.instructions ? 'border-red-400/50' : 'border-border'
           )}
         />
@@ -221,7 +221,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
           onChange={(e) => setHelpText(e.target.value)}
           placeholder="Additional context or tips..."
           rows={3}
-          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20"
+          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20"
         />
       </div>
 
@@ -296,7 +296,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
           id="category"
           value={categoryId}
           onChange={(e) => setCategoryId(e.target.value)}
-          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20"
+          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20"
         >
           <option value="">None</option>
           {categories.map(cat => (
@@ -318,7 +318,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
             onChange={(e) => setTagInput(e.target.value)}
             onKeyDown={handleTagInputKeyDown}
             placeholder="Type tag and press Enter"
-            className="flex-1 rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20"
+            className="flex-1 rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20"
           />
           <button
             type="button"
@@ -359,7 +359,7 @@ export function StepForm({ onSubmit, onCancel, initialData, submitLabel, isSubmi
           id="visibility"
           value={visibility}
           onChange={(e) => setVisibility(e.target.value as 'private' | 'team' | 'public')}
-          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20"
+          className="w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20"
         >
           <option value="private">Private (only me)</option>
           <option value="team">Team (my team members)</option>