feat(welcome): show first-run welcome dialog on fresh install

Introduce a one-time welcome dialog that explains CC Switch's workflow
to new users: how their existing config is preserved as a "default"
provider and how the bundled "Official" preset enables one-click revert.
Upgrade users are excluded by checking is_providers_empty() at startup
and never see the dialog.

Persistence follows the existing *_confirmed convention in AppSettings
(proxy/usage/stream_check/failover), stored in settings.json. The field
is only written when the user explicitly clicks the confirm button,
keeping its semantics strictly about user acknowledgement.

Also adds two reusable DAO helpers:
- Database::is_providers_empty for fresh-install detection, using
  EXISTS(SELECT 1) for a short-circuit query.
- Database::get_bool_flag accepting "true" | "1", with
  init_default_official_providers migrated to use it.

Dialog copy in zh/en/ja uses conditional phrasing so it stays
accurate whether or not existing live config was found.

src/App.tsx

@@ -62,6 +62,7 @@ import PromptPanel from "@/components/prompts/PromptPanel";
 import { SkillsPage } from "@/components/skills/SkillsPage";
 import UnifiedSkillsPanel from "@/components/skills/UnifiedSkillsPanel";
 import { DeepLinkImportDialog } from "@/components/DeepLinkImportDialog";
+import { FirstRunNoticeDialog } from "@/components/FirstRunNoticeDialog";
 import { AgentsPanel } from "@/components/agents/AgentsPanel";
 import { UniversalProviderPanel } from "@/components/universal";
 import { McpIcon } from "@/components/BrandIcons";
@@ -1354,6 +1355,7 @@ function App() {
       />
 
       <DeepLinkImportDialog />
+      <FirstRunNoticeDialog />
     </div>
   );
 }