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/timeline/queries.ts

@@ -210,6 +210,7 @@ export function getAllProjects(db: Database): string[] {
     SELECT DISTINCT project
     FROM sdk_sessions
     WHERE project IS NOT NULL AND project != ''
+      AND project != 'observer-sessions'
     ORDER BY project ASC
   `);