fix(logs): harden global logs with shared parsing, SSE fixes, and level filter (#539)

- Extract log parsing utilities (normalizeContainerName, parseLogTimestamp,
  detectLogLevel, stripControlChars, demuxDockerLog) to shared module,
  eliminating duplication between polling and SSE endpoints
- Fix timestamp regex to accept timezone offsets (+HH:MM/-HH:MM), not just Z
- Replace any[] with typed GlobalLogEntry interface
- Add SSE heartbeat (30s) to prevent reverse proxy timeouts
- Add X-Accel-Buffering: no header for nginx SSE compatibility
- Replace swallowed catch blocks with console.warn diagnostics
- Fix SSE not reconnecting on node switch (missing dep in effect array)
- Add settings change event so devMode/pollRate updates apply without remount
- Add log level filter (ALL/ERROR/WARN/INFO) to toolbar
- Replace overflow-auto div with ScrollArea for design system compliance
- Add strokeWidth={1.5} to all toolbar icons
- Include stack name in download format
- Surface fetch errors with inline banner
- Add 39 unit tests for all log parsing utilities
- Update docs with level filter documentation and refreshed screenshot

frontend/src/lib/events.ts

@@ -6,3 +6,9 @@ export interface SenchoOpenLogsDetail {
   containerId: string;
   containerName: string;
 }
+
+export const SENCHO_SETTINGS_CHANGED = 'sencho-settings-changed';
+
+export interface SenchoSettingsChangedDetail {
+  changedKeys: string[];
+}