feat(models): add Z.AI (GLM) credential integration and setup flow

Implement first-class Z.AI credential storage and authentication:

- New auth provider: src/auth/zai.ts for Z.AI API key management
- New CLI command: flynn zai-auth to store Z.AI API keys
- New TUI command: /login zai for interactive credential entry
- Modified src/auth/index.ts to register zai provider
- Modified src/cli/index.ts to register zai-auth command
- Modified src/cli/setup/providers.ts to include Z.AI in setup wizard
- Modified src/daemon/models.ts to support zhipuai use_oauth flag
- Modified src/daemon/clientFactory.test.ts to add Z.AI tests
- Modified src/frontends/tui/commands.ts to add login command
- Modified src/frontends/tui/minimal.ts to support credential prompts

This allows users to authenticate with Z.AI (GLM models) without
embedding secrets in config files. Credentials are stored securely in
~/.config/flynn/auth.json and resolved at runtime.

Updated state.json with new feature entry documenting the integration.

src/frontends/tui/commands.ts

@@ -124,7 +124,7 @@ Commands:
   /model [name]          Show or switch model tier (local, default, fast, complex)
   /model <tier> <p/m>    Change tier's provider/model (e.g. /model default anthropic/claude-sonnet-4)
   /backend [provider]    Show or switch local backend (ollama, llamacpp)
-  /login [provider]      Authenticate with GitHub or OpenAI
+  /login [provider]      Authenticate with GitHub, OpenAI, or Z.AI
   /pair                  List pending pairing codes and approved senders
   /pair generate [label] Generate a new DM pairing code
   /pair revoke <ch> <id> Revoke an approved sender
@@ -178,7 +178,7 @@ export const COMMAND_TOOLTIPS: Record<string, string> = {
   '/status': 'Show session info and token usage',
   '/fullscreen': 'Switch to fullscreen mode',
   '/fs': 'Switch to fullscreen mode',
-  '/login': 'Authenticate with GitHub or OpenAI (OAuth device flow)',
+  '/login': 'Authenticate with GitHub/OpenAI (OAuth) or Z.AI (API key store)',
   '/pair': 'Generate/list/revoke DM pairing codes',
   '/transfer': 'Transfer session to another frontend',
   '/quit': 'Exit TUI',