feat(activity-log): phase 6 - dashboard widget + settings panel + docs

- Dashboard 'Recent Activity' widget: latest 5 entries, live prepend, 'View all' -> Activity tab
- Settings 'Activity Log' panel: retention (enabled/max_days/max_entries) GET/PUT, clear (confirm + auth-required toast), CSV/JSON export
- audit-log vs ephemeral debug Log Viewer distinction note + cross-links
- public helpers fetchRecentEntries/renderCompactEntry on activity-log.ts (reused, no dup markup)
- README Activity Log section; i18n across en/ru/zh
- review fixes: clear 401 surfaces toast; empty widget transitions on first live event

README.md

@@ -82,6 +82,19 @@ LedGrab speaks many protocols, so a single setup can drive everything from a DIY
 - Real-time FPS, latency, and uptime charts
 - Localized in English, Russian, and Chinese
 
+### Activity Log
+
+The **Activity** tab is a persistent, queryable audit log of everything LedGrab has done — entity changes, auth events, device connections, and system actions.
+
+- Filter by category (auth, device, entity, capture, system), severity, actor, entity type, date range, or free text
+- Live-append of new events as they happen
+- Export as CSV or JSON (authentication required)
+- Entity crosslinks navigate directly to the relevant card
+- **Retention settings** (Settings → Activity Log): configure max age, max entry count, and toggle recording on/off
+- **Clear log** (Settings → Activity Log, requires authentication) — audited: a system entry records who cleared the log and when
+
+> **Note:** The Activity Log is distinct from the **debug Log Viewer** (Settings → General → Open Log Viewer). The Log Viewer is an ephemeral real-time tail of the server's Python log stream (WARNING/ERROR lines, resets on disconnect). The Activity Log is a structured, persistent SQLite-backed record of semantic application events.
+
 ### Home Assistant Integration
 
 - HACS-compatible custom component (separate repository)