feat(dashboard): add working indicator for active sessions

Visual feedback when an agent is actively processing:

1. **Spinner on status dots** (SessionCard.js, Modal.js)
   - Status dot gets a spinning ring animation when session is active/starting
   - Uses CSS border trick with transparent borders except top

2. **Working indicator in chat** (ChatMessages.js, Modal.js)
   - Shows at bottom of conversation when agent is working
   - Bouncing dots animation ("...") next to "Agent is working" text
   - Only visible for active/starting statuses

3. **CSS animations** (styles.css)
   - spin-ring: 0.8s rotation for the status dot border
   - bounce-dot: staggered vertical bounce for the working dots

4. **Status metadata** (status.js)
   - Added `spinning: true` flag for active and starting statuses
   - Used by components to conditionally render spinner elements

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

dashboard/utils/status.js

@@ -22,6 +22,7 @@ export function getStatusMeta(status) {
         dot: 'bg-active',
         badge: 'bg-active/18 text-active border-active/40',
         borderColor: '#5fd0a4',
+        spinning: true,
       };
     case 'starting':
       return {
@@ -29,6 +30,7 @@ export function getStatusMeta(status) {
         dot: 'bg-starting',
         badge: 'bg-starting/18 text-starting border-starting/40',
         borderColor: '#7cb2ff',
+        spinning: true,
       };
     case 'done':
       return {