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/web/server.ts

@@ -539,6 +539,7 @@ export class WebServer extends EventEmitter {
       // Fresh, not the 2s cache: the settings PUT does not invalidate it, and a
       // flip must apply to the very next spawn.
       childrenFollowParent: async () => (await this.readSettings(true)).spawnedTabsFollowParent === true,
+      caseDefaults: this.store,
     });
     if (this.testMode) this.tabLayouts.markRestorationSkipped();
 
@@ -794,8 +795,8 @@ export class WebServer extends EventEmitter {
   /**
    * Add a tentative session only after its owner layout accepts the creation.
    * The session enters the live map inside the layout service's owner lock, so
-   * the placement it asked for (`tabGroupId`) lands in the same versioned write
-   * that first records it.
+   * the placement it asked for (`tabGroupId`, else its case's default group)
+   * lands in the same versioned write that first records it.
    */
   private async registerSessionWithLayout(session: Session, placement?: SessionPlacementHint): Promise<TabLayout> {
     return this.tabLayouts.sessionCreated(ownerLayoutKey(session.owner), session.id, placement, () => {