OAuth credential flow for MCP servers

## Goal
Users can connect OAuth-protected MCP servers (e.g., GitHub, Google) via a PKCE popup. Per-user tokens are stored in DynamoDB and injected at invocation time.

## Acceptance criteria
- An MCP server with `oauthClientId` set shows a "Connect" button in the management UI
- Clicking "Connect" opens a PKCE popup via `web/lib/mcp-auth.ts`
- After authorization, tokens are saved to a `McpServerCredential` record (owner-only access)
- At chat invocation time, the user's access token is retrieved and injected as the `Authorization` header in the `remote_mcp` tool spec
- If the token is expired, the UI prompts re-authorization before invoking

## Files to modify
- `web/app/(with-auth)/agents/page.tsx` — add Connect button
- `web/lib/mcp-auth.ts` — ensure token storage writes to `McpServerCredential` via AppSync
- `web/lib/agentcore-transport.ts` — retrieve `McpServerCredential` at invoke time and inject token

## Dependencies
- Issue #148 (MCP CRUD UI must exist first)

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*Imported from `waltmayf/agentcore-amplify-fullstack`#35 — originally filed by @waltmayf on 2026-07-02. Migrated to open-source repo; cross-references updated post-import.*

Read issue #148 first, then inspect web/app/(with-auth)/agents/page.tsx, web/lib/mcp-auth.ts, and web/lib/agentcore-transport.ts. Trace the existing AppSync and MCP invocation paths before implementing the Connect flow and credential lookup. Done means OAuth-protected servers can authorize, store owner-only credentials, inject the user's token, and prompt again when it expires.

Written by the indexing model from the issue text.