fix: update workbar tab titles when switching language

Derive application and default terminal titles from the current locale while preserving renamed and unmarked legacy terminal titles. Add locale-switch and layout-restoration regression coverage.

File-level scope also includes existing browser popup selection and close handling in the sidebar, its regression test, and workbar documentation updates.

Validation: 74 focused tests passed; typecheck and lint passed. Full test runs timed out with failures in settings navigation, Agent packaging, and database migration tests. Electron verification remains outstanding.

Release note: Workbar application and new default terminal titles now follow the interface language, including after restoring a layout; custom and legacy terminal names are preserved.

src/shared/workbar-contracts.ts

@@ -199,6 +199,7 @@ export const workbarTabInstanceSchema = z
     id: z.string().uuid(),
     appId: workbarAppIdSchema,
     title: boundedUtf8TextSchema(WORKBAR_LIMITS.maximumTitleBytes),
+    defaultTitleNumber: z.number().int().positive().optional(),
     targetRef: workbarTargetRefSchema.optional()
   })
   .strict()