Replace freetext email inputs with a user lookup for tag/assign/reallocate

Typo'd addresses in the old <input type="email"> fields silently matched
nobody (assigned_to/task_tags.user_email have no FK), so a task could vanish
into an unreachable owner with no error. New UsersContext + UserPicker pull
the real user list from central auth (/api/auth/users?app=tasks), same
pattern maintenance already uses for its assignee dropdown.

frontend/src/types.ts

@@ -141,6 +141,13 @@ export interface User {
   caps: string[]   // bare slugs — verify?app=tasks strips the prefix
 }
 
+// Users with access to this app, served by the central auth service.
+export interface AuthUser {
+  id: number
+  email: string
+  name: string
+}
+
 export function can(user: User, cap: string): boolean {
   return user.is_admin || user.caps.includes(cap)
 }