chore(hygiene): Phase 4 guardrails — noUnusedLocals/Parameters + dead-code sweep

Enable tsconfig noUnusedLocals + noUnusedParameters (permanent guardrail: every tsc/
build now flags re-accumulated dead code). Fixed the 8 violations that surfaced, all
genuine dead code:
- Sidebar: deleted unused SORT_IDS/SHOW_IDS/rollSeed consts + the dead Toggle component
  (+ its now-unused Switch import).
- PlexBrowse: dropped the unused onClearSearch prop (Props + destructure + App call site).
- VideoEditor: dropped the unused map index; notifications: deleted the dead back-compat
  toast(); storage: deleted the unused non-account usePersistedState (+ readJSON import).
- SavedViewsWidget: deleted the dead loadDefaultViewFilters export (App loads the default
  view inline; resolves the last knip unused-export → knip now clean).

Backend: added backend/ruff.toml (ignore E402 — intentional pre-import setup in main.py)
and fixed the 6 E702/E741 style items (_vtt_s_to_ts semicolons, ambiguous `l` loop vars)
so ruff is green. localdev boots; Feed/Plex/sidebar render; 0 console errors.

backend/app/routes/plex.py

@@ -1346,11 +1346,15 @@ def _vtt_ts_to_s(ts: str) -> float:
 
 def _vtt_s_to_ts(x: float) -> str:
     x = max(0.0, x)
-    h = int(x // 3600); x -= h * 3600
-    m = int(x // 60); x -= m * 60
-    s = int(x); ms = int(round((x - s) * 1000))
+    h = int(x // 3600)
+    x -= h * 3600
+    m = int(x // 60)
+    x -= m * 60
+    s = int(x)
+    ms = int(round((x - s) * 1000))
     if ms >= 1000:
-        s += 1; ms -= 1000
+        s += 1
+        ms -= 1000
     return f"{h:02d}:{m:02d}:{s:02d}.{ms:03d}"