Merge pull request #676: feat(tabs): per-case default session group

Conflict in CLAUDE.md's State Files line resolved by keeping #661's new
config-backup entries and this PR's "case default groups" in state.json.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/types/app-state.ts

@@ -121,6 +121,12 @@ export interface AppState {
   sessionOrder?: string[];
   /** Owner-scoped authoritative grouped tab layouts. */
   tabLayouts?: Record<string, TabLayout>;
+  /**
+   * Per-case default session group, keyed by the tab-layout owner key, then case
+   * name. A group id from that owner's layout, or null for an intentional
+   * Ungrouped choice. Read and written only through TabLayoutService.
+   */
+  caseDefaultGroups?: Record<string, Record<string, string | null>>;
 }
 
 // ========== Default Configuration ==========