Fix icon picker positioning + match input height + add overlay rule

- IconPicker: use position:fixed with getBoundingClientRect() for
  dropdown (fixes rendering at page footer instead of below button)
- Match icon button height to text input (py-2 same as inputs)
- CLAUDE.md: add rule about overlays requiring position:fixed with
  inline styles (Tailwind v4 classes unreliable in flex containers)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -39,6 +39,8 @@ The README is the primary user-facing documentation and must accurately reflect
 3. Start: `cd <repo_root> && IMMICH_WATCHER_DATA_DIR=./test-data IMMICH_WATCHER_SECRET_KEY=test-secret-key-minimum-32chars nohup python -m uvicorn immich_watcher_server.main:app --host 0.0.0.0 --port 8420 > /dev/null 2>&1 &`
 4. Verify: `curl -s http://localhost:8420/api/health`
 
+**IMPORTANT**: Overlays (modals, dropdowns, pickers) MUST use `position: fixed` with inline styles and `z-index: 9999`. Tailwind CSS v4 `fixed`/`absolute` classes do NOT work reliably inside flex/overflow containers in this project. Always calculate position from `getBoundingClientRect()` for dropdowns, or use `top:0;left:0;right:0;bottom:0` for full-screen backdrops.
+
 **IMPORTANT**: When the user requests it, restart the frontend dev server:
 1. Kill existing process on port 5173
 2. Start: `cd frontend && npx vite dev --port 5173 --host &`