feat(providers): seed an official preset on startup for Claude/Codex/Gemini

New and existing users now see a built-in "Claude Official" / "OpenAI
Official" / "Google Official" entry in their provider list, so switching
back to the official endpoint is one click away instead of buried in the
README.

- New providers_seed.rs holds the three seeds (id, name, settings_config,
  icon) keyed by AppType, with a single is_official_seed_id() helper that
  scans OFFICIAL_SEEDS so the id list has one source of truth.
- Database::init_default_official_providers() runs once per database
  (gated by an official_providers_seeded setting flag), appends each seed
  to the end of the sort order, and never touches is_current.
- Startup also auto-imports the live config (settings.json / auth.json /
  .env) as a "default" provider before seeding, so users with an existing
  manual config don't lose it when they click the official preset.
- Database::has_non_official_seed_provider() replaces the get_all_providers
  call in import_default_config's gating check with an id-only scan,
  dropping the N+1 endpoint sub-queries from every startup.

src-tauri/src/database/dao/mod.rs

@@ -6,6 +6,7 @@ pub mod failover;
 pub mod mcp;
 pub mod prompts;
 pub mod providers;
+pub mod providers_seed;
 pub mod proxy;
 pub mod settings;
 pub mod skills;