refactor: fix and tighten mechanical lint rules

- no-useless-escape: drop needless backslashes in character classes and
  URL/path regexes (gh-cli, store, task, modelFilter, useFileMention,
  RoutineEditor, ScheduleForm).
- no-case-declarations: wrap case bodies in ProjectOverview and
  SettingsModal with block scopes.
- prefer-const: convert a never-reassigned slug binding in agent-import;
  annotate legitimate forward-declared let bindings in dashboard.ts that
  callbacks close over before assignment.
- no-fallthrough: add missing break after settings-subcommand error.
- no-empty-interface/no-empty-object-type: convert ProjectManifest from
  empty interface extension to a type alias.
- no-unused-expressions: replace `x && x.method()` short-circuits in
  TerminalModal with optional chaining.

Then ratchet these rules from warn → error so regressions are blocked.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/SettingsModal.tsx

@@ -3024,7 +3024,7 @@ export function SettingsModal({
         );
       case "pi-extensions":
         return <PiExtensionsManager addToast={addToast} projectId={projectId} />;
-      case "authentication":
+      case "authentication": {
         // CLI-backed providers (currently just claude-cli) render their own
         // compact card with Enable/Disable + Test actions — bypassing the
         // OAuth/API-key rendering below. Filter them out of the standard
@@ -3231,6 +3231,7 @@ export function SettingsModal({
             )}
           </>
         );
+      }
     }
   };