Feature: ThinkReview enterprise self-hosted gateway option

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Summary

Add support for ThinkReview Self-Hosted Gateway as an AI provider option, allowing Teams subscribers to route code review traffic to an on-prem enterprise gateway while keeping sign-in, agents, and billing on ThinkReview cloud.

Motivation

Enterprise customers on the Teams plan need the ability to run AI-powered code reviews on their own infrastructure with their own LLM, keeping code within their network.

Changes

New self-hosted gateway provider

• Add ThinkReview Self-Hosted Gateway as a selectable AI provider in the extension popup (Teams plan only)
• New utils/enterprise-gateway.js module with:
  • Teams subscription validation (canUseEnterpriseGatewayFromStorage)
  • Gateway base URL normalization (normalizeGatewayBaseUrl)
  • Pre-flight checks before routing review traffic (assertSelfHostedGatewayReady)

Popup UI

• New provider card for self-hosted gateway with Teams badge
• Configuration panel with gateway base URL input, Test Connection, and Save Settings buttons
• Provider visibility gated on Teams subscription status
• Auto-migrate provider selection when gateway URL is configured

API routing

• CloudService.getReviewApiBaseUrl() routes patch review and conversational review to the configured gateway when aiProvider is self-hosted
• background.js gates conversational and patch review requests through assertSelfHostedGatewayReady
• Review endpoints (reviewPatchCode_1_1, getConversationalReview_1_1) use dynamic base URL

Portal auth hardening

• content-webapp-auth.js guards against invalidated extension context after reload/update
• Prevents errors when syncing portal sign-in with the extension after an update

Acceptance criteria

• Teams subscribers see the Self-Hosted Gateway provider option in popup settings
• Non-Teams users do not see or cannot select the self-hosted provider
• Gateway base URL can be configured, tested, and saved
• Patch review and conversational review route to the configured gateway URL when self-hosted is selected
• Clear error messages when gateway is not configured or user lacks Teams subscription
• Portal auth sync degrades gracefully after extension updates

Related pull request

Implemented in #285