feat: add xAI as official provider (#865)

* feat: add xAI as official provider

- Add xAI preset to ProviderManager (alphabetical order)
- Add xAI provider detection via XAI_API_KEY
- Add xAI startup screen heuristic (x.ai base URL or grok model)
- Add xAI status display properties
- Add grok-4 and grok-3 context windows
- Add xAI model fallbacks across all tiers
- Fix JSDoc priority order in providerAutoDetect

Co-Authored-By: Claude Opus 4.6 <noreply@openclaude.dev>

* fix(xai): persist relaunch classification for xAI profiles

Addresses reviewer feedback on feat/xai-official-provider:
- isProcessEnvAlignedWithProfile now validates XAI_API_KEY for x.ai
  base URLs, mirroring the Bankr pattern. Without this, relaunch
  skips re-applying the profile, XAI_API_KEY stays unset, and
  getAPIProvider() falls back to 'openai'.
- buildOpenAICompatibleStartupEnv now sets XAI_API_KEY when syncing
  active xAI profile to the legacy fallback file.
- Adds 'xai' to VALID_PROVIDERS and --provider xai CLI flag support.
- Adds xAI detection to providerDiscovery label heuristics.
- Adds 'xai' to legacy ProviderProfile type/isProviderProfile guard.
- Adds targeted tests for relaunch alignment, flag application, and
  discovery labeling.

Co-Authored-By: OpenClaude <openclaude@gitlawb.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@openclaude.dev>
Co-authored-by: OpenClaude <openclaude@gitlawb.com>

src/components/ProviderManager.tsx

@@ -1427,6 +1427,11 @@ export function ProviderManager({ mode, onDone }: Props): React.ReactNode {
         label: 'Together AI',
         description: 'Together chat/completions endpoint',
       },
+      {
+        value: 'xai',
+        label: 'xAI',
+        description: 'xAI Grok OpenAI-compatible endpoint',
+      },
       {
         value: 'zai',
         label: 'Z.AI - GLM Coding Plan',