refactor: assistant page — TaskLane UX + ChatSidebar improvements

TaskLane:
- Simplify action buttons: merge "Paste Result"/"Type Answer" into single
  primary button, make "Skip" icon-only (reduces decision points from 3→1)
- Strengthen done state: solid left-border success green + checkmark icon
  instead of faint tint that nearly disappears
- Boost progress bar: 3px→5px, better contrast colors, inline count label
- Differentiate from ChatSidebar: use bg-page instead of bg-sidebar,
  add accent top-border to signal "active workspace"
- Make skipped tasks clickable to un-skip (matching done→reopen pattern)
- Fix slide-in animation: add slide-in-from-right keyframe
- Fix duplicate style props, stray quote from replace_all
- Consolidate 6 grip dot divs to Array.from loop

ChatSidebar:
- Add inline delete confirmation ("Delete? Yes / No") instead of
  immediate destructive action
- Fix text-xs text-[0.625rem] double class conflict on Pinned header

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

frontend/src/index.css

@@ -82,6 +82,7 @@
   --animate-fade-in: fade-in 200ms ease-out both;
   --animate-fade-in-up: fade-in-up 200ms ease-out both;
   --animate-slide-in-left: slide-in-from-left 200ms ease-out;
+  --animate-slide-in-right: slide-in-from-right 200ms ease-out both;
   --animate-slide-in-bottom: slide-in-from-bottom 200ms ease-out both;
   --animate-scale-in: scale-in 150ms ease-out both;
   --animate-fade: fadeIn 300ms ease both;
@@ -95,6 +96,9 @@
   @keyframes slide-in-from-left {
     from { transform: translateX(-100%); } to { transform: translateX(0); }
   }
+  @keyframes slide-in-from-right {
+    from { opacity: 0; transform: translateX(16px); } to { opacity: 1; transform: translateX(0); }
+  }
   @keyframes slide-in-from-bottom {
     from { opacity: 0; transform: translateY(16px); } to { opacity: 1; transform: translateY(0); }
   }