0.5.5: inline lock toggle, proper lock indentation

- Track lock becomes a small inline icon toggle sitting next to the
  track label, replacing the static badge and removing the context
  menu's lock/unlock entry.
- Locked tracks are fully inert: the lane and name have pointer-events
  disabled, only the lock button itself stays clickable. The pile of
  hover-neutralizing CSS rules is gone.
- Lock button is icon-only (no background/border box) and swaps to the
  opposite icon on hover (filled unlock when hovering a locked track,
  so the click affordance is obvious).
- Locked tracks in groups keep their indentation — the drag-handle
  switches from display:none to visibility:hidden so the reserved
  space still aligns with unlocked sibling layers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "animatr",
   "private": true,
-  "version": "0.5.4",
+  "version": "0.5.5",
   "type": "module",
   "scripts": {
     "dev": "vite",