feat: per-provider options (thinking mode, 1M context) and SDK betas integration

- Add ProviderOptionsSection component for per-provider thinking_mode and context_1m
- Store options in options_json column (DB providers) or settings table (env provider)
- Add /api/providers/options GET/PUT route
- Fix SDK integration: use `betas` (not `betaHeaders`) for context-1m-2025-08-07
- ChatView and /chat page always reset thinkingMode on provider change (isolation fix)
- Auto-correct model when switching providers if model not in new provider's list
- Add i18n keys for provider options in both en.ts and zh.ts
- Update test fixtures with options_json field
- Remove unused lastAssistantText variable in claude-client.ts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/providers/options/route.ts

@@ -0,0 +1,40 @@
+import { NextRequest, NextResponse } from 'next/server';
+import { getProviderOptions, setProviderOptions } from '@/lib/db';
+import type { ProviderOptions } from '@/types';
+
+/**
+ * GET /api/providers/options?providerId=xxx
+ * Returns per-provider options (thinking_mode, context_1m).
+ */
+export async function GET(request: NextRequest) {
+  const providerId = request.nextUrl.searchParams.get('providerId') || 'env';
+  const options = getProviderOptions(providerId);
+  return NextResponse.json({ options });
+}
+
+/**
+ * PUT /api/providers/options
+ * Update per-provider options. Body: { providerId, options: { thinking_mode?, context_1m? } }
+ */
+export async function PUT(request: NextRequest) {
+  try {
+    const body = await request.json();
+    const { providerId = 'env', options } = body as { providerId?: string; options: ProviderOptions };
+
+    if (!options || typeof options !== 'object') {
+      return NextResponse.json({ error: 'Invalid options' }, { status: 400 });
+    }
+
+    // Merge with existing options (partial update)
+    const existing = getProviderOptions(providerId);
+    const merged: ProviderOptions = { ...existing, ...options };
+    setProviderOptions(providerId, merged);
+
+    return NextResponse.json({ options: merged });
+  } catch (error) {
+    return NextResponse.json(
+      { error: error instanceof Error ? error.message : 'Failed to update options' },
+      { status: 500 },
+    );
+  }
+}