feat(ui): hide observer-sessions project from UI lists

Observer sessions (internal SDK-driven worker queries) run under a
synthetic project name 'observer-sessions' to keep them out of
claude --resume. They were still surfacing in the viewer project
picker and unfiltered observation/summary/prompt feeds.

Filter them out at every UI-facing query:
- SessionStore.getAllProjects and getProjectCatalog
- timeline/queries.ts getAllProjects
- PaginationHelper observations/summaries/prompts when no project is selected

When a caller explicitly requests project='observer-sessions',
results are still returned (not a hard ban, just hidden by default).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/services/sqlite/SessionStore.ts

@@ -1192,6 +1192,7 @@ export class SessionStore {
       SELECT DISTINCT project
       FROM sdk_sessions
       WHERE project IS NOT NULL AND project != ''
+        AND project != 'observer-sessions'
     `;
     const params: unknown[] = [];
 
@@ -1218,6 +1219,7 @@ export class SessionStore {
         MAX(started_at_epoch) as latest_epoch
       FROM sdk_sessions
       WHERE project IS NOT NULL AND project != ''
+        AND project != 'observer-sessions'
       GROUP BY COALESCE(platform_source, '${DEFAULT_PLATFORM_SOURCE}'), project
       ORDER BY latest_epoch DESC
     `).all() as Array<{ platform_source: string; project: string; latest_epoch: number }>;