feat: implement all P0 UX improvements - visual hierarchy, readable previews, search filter, default session

P0.1: Visual Hierarchy for Tiles
- Backend: Added last_activity_at to GET /api/sessions by querying tmux list-sessions -F '#{session_activity}'
  Updated sessions.py with tuple return from enumerate_sessions and activity cache
  Updated main.py poll loop to populate both session endpoints with activity data
- Frontend: sessionPriority() now returns 'active' (< 5 min) in addition to 'bell'/'idle'
  Added .session-tile--active (accent left border) and .session-tile--stale (dimmed styling) CSS classes
  Updated session-tile.js and sidebar-item.js to apply priority-based classes
- Fixed P4 hover fade bug: removed opacity:0 rule on .session-tile:hover .tile-meta

P0.2: Readable Tile Previews
- Wired --preview-font-size CSS variable (was set by JS but never consumed by CSS)
  Changed default from 11px to 12px with line-height 1.2 (was 1.0)
- Reduced preview line count from 20 to 12 lines for better readability at larger size

P0.3: Search/Filter Bar
- New <search-filter> Lit component with live input, match count, and clear button
  Replaces empty filter-bar div in index.html with functional search component
  Integrated into renderGrid() for live filtering and bindStaticEventListeners() for re-render on input

P0.4: Default Session / Quick-Resume
- Updated restoreState() to fall back to default_session setting when no active_session in server state
  Added existence check before opening to prevent errors on non-existent sessions

Files changed: sessions.py, main.py, app.js, style.css, session-tile.js, sidebar-item.js, utils.js, index.html
New files: search-filter.js (Lit component)
Test updates: test_api.py, test_frontend_css.py, test_integration.py, test_sessions.py

All 1306 tests pass.

Generated with Amplifier

Co-Authored-By: Amplifier <240397093+microsoft-amplifier@users.noreply.github.com>

muxplex/main.py

@@ -51,6 +51,7 @@ from muxplex.auth import (
 from muxplex.bells import apply_bell_clear_rule, process_bell_flags
 from muxplex.sessions import (
     enumerate_sessions,
+    get_session_activity,
     get_session_list,
     get_snapshots,
     run_tmux,
@@ -180,12 +181,12 @@ async def _run_poll_cycle() -> None:
     global _settings_sync_counter
     async with state_lock:
         # 1. Enumerate live tmux sessions
-        names = await enumerate_sessions()
+        names, activity = await enumerate_sessions()
         name_set = set(names)
 
         # 2. Capture pane snapshots and update in-memory snapshot cache
         new_snapshots = await snapshot_all(names)
-        update_session_cache(names, new_snapshots)
+        update_session_cache(names, new_snapshots, activity)
 
         # 3. Load current persisted state
         state = load_state()
@@ -583,9 +584,10 @@ async def patch_state(patch: StatePatch) -> dict:
 
 @app.get("/api/sessions")
 async def get_sessions() -> list[dict]:
-    """Return list of sessions with name, snapshot, and bell data."""
+    """Return list of sessions with name, snapshot, bell, and activity data."""
     names = get_session_list()
     snapshots = get_snapshots()
+    activities = get_session_activity()
     state = await read_state()
 
     result = []
@@ -597,6 +599,7 @@ async def get_sessions() -> list[dict]:
                 "name": name,
                 "snapshot": snapshots.get(name, ""),
                 "bell": bell,
+                "last_activity_at": activities.get(name),
             }
         )
     return result
@@ -654,7 +657,7 @@ async def create_session(payload: CreateSessionPayload) -> dict:
             # Some commands (amplifier-workspace) create the session then
             # try to attach (which fails without a TTY).  If the session
             # exists despite the non-zero exit, treat it as success.
-            sessions = await enumerate_sessions()
+            sessions, _act = await enumerate_sessions()
             if name in sessions:
                 _log.info(
                     "Session command exited %d but session '%s' exists -- "
@@ -699,9 +702,9 @@ async def create_session(payload: CreateSessionPayload) -> dict:
     # Eagerly refresh the session cache so the next GET /api/sessions
     # reflects the newly-created session without waiting for the poll loop.
     try:
-        fresh_names = await enumerate_sessions()
+        fresh_names, fresh_activity = await enumerate_sessions()
         fresh_snapshots = await snapshot_all(fresh_names)
-        update_session_cache(fresh_names, fresh_snapshots)
+        update_session_cache(fresh_names, fresh_snapshots, fresh_activity)
     except Exception:
         pass  # non-fatal; poll loop will catch up
 
@@ -1349,6 +1352,7 @@ async def federation_sessions(request: Request) -> list[dict]:
     # Build local sessions with deviceId/deviceName/remoteId/sessionKey tags
     names = get_session_list()
     snapshots = get_snapshots()
+    activities = get_session_activity()
     state = await read_state()
     local_sessions: list[dict] = []
     for name in names:
@@ -1359,6 +1363,7 @@ async def federation_sessions(request: Request) -> list[dict]:
                 "name": name,
                 "snapshot": snapshots.get(name, ""),
                 "bell": bell,
+                "last_activity_at": activities.get(name),
                 "deviceId": local_device_id,
                 "deviceName": local_device_name,
                 "remoteId": None,