feat(oauth): browser-initiated dynamic client registration (DCR)

Adds an OAuth 2.1 public client registration flow so MCP clients can
self-register via a user-facing consent page instead of requiring manual
setup in Settings.

Server:
- DB migration adds `is_public` and `created_via` columns to oauth_clients
- New GET /api/oauth/register/validate — validates DCR params, returns
  requested scopes; unauthenticated callers get loginRequired flag
- New POST /api/oauth/register — creates a public client, saves consent,
  and redirects with client_id (cookie auth required)
- `authenticateClient` / `refreshTokens` skip secret check for public
  clients (PKCE provides the security guarantee)
- `createOAuthClient` accepts options for isPublic/createdVia; public
  clients store an opaque secret hash instead of a usable secret
- `rotateOAuthClientSecret` blocked on public clients
- `isValidRedirectUri` extracted as a shared helper
- Discovery metadata now advertises registration_endpoint and auth method
  `none`; token/revoke endpoints no longer require client_secret for
  public clients

Client:
- New OAuthRegisterPage (/oauth/register) — loading → optional
  login-required gate → scope selection → done states
- New ScopeGroupPicker component — collapsible groups, indeterminate
  checkboxes, select-all per group or globally
- oauthApi.register.{validate,submit} added to api/client.ts
- apiClient exported so it can be reused outside api/client.ts
- IntegrationsTab tests fixed for new collapsible section structure
- collab_notes fallback changed from undefined to [] in MCP trip tools

client/src/i18n/translations/de.ts

@@ -228,6 +228,7 @@ const de: Record<string, string | { name: string; category: string }[]> = {
   'settings.mcp.endpoint': 'MCP-Endpunkt',
   'settings.mcp.clientConfig': 'Client-Konfiguration',
   'settings.mcp.clientConfigHint': 'Ersetze <your_token> durch ein API-Token aus der Liste unten. Der Pfad zu npx muss ggf. für dein System angepasst werden (z. B. C:\\PROGRA~1\\nodejs\\npx.cmd unter Windows).',
+  'settings.mcp.clientConfigHintOAuth': 'Replace <your_client_id> and <your_client_secret> with the credentials shown in the OAuth 2.1 client you created above. mcp-remote will open your browser to complete the authorization the first time you connect. The path to npx may need to be adjusted for your system (e.g. C:\PROGRA~1\nodejs\npx.cmd on Windows).',
   'settings.mcp.copy': 'Kopieren',
   'settings.mcp.copied': 'Kopiert!',
   'settings.mcp.apiTokens': 'API-Tokens',