feat(mcp): add browser OAuth for remote servers

[chengyongru]

Summary

• add browser-based OAuth for remote Streamable HTTP and SSE MCP servers using the official MCP SDK
• add one-click Xmind, Notion, and Linear presets, plus OAuth support for imported/custom MCP configs
• let custom HTTP/SSE servers choose None, OAuth, or Headers directly in the Apps UI
• make the Apps catalog explicitly describe MCP tools and show clear Connect / Connecting / Connected states
• persist scoped MCP credentials outside config.json and hot-reload tools after authorization
• register nanobot branding metadata so provider consent pages can show the project icon and website
• support OAuth from a remotely accessed plain-HTTP WebUI with a guided full-callback-URL paste flow

UX

The primary flow is Apps -> MCP -> Connect -> provider consent.

• HTTPS and localhost WebUIs return to nanobot automatically without copying anything
• a remote plain-HTTP WebUI registers a loopback callback, explains that the localhost page will not
  load, and presents one field for pasting the complete URL from the browser address bar; users do
  not need to identify or extract the authorization code
• an explicit Cancel action remains available throughout authorization, including after the user
  closes the OAuth popup
• popup recovery appears only when popup creation is actually blocked; COOP-isolated login tabs are
  no longer mistaken for closed popups
• the catalog, active connection controls, callback form, and custom MCP form remain within bounds
  from 320px upward; wider two-column layouts only activate when the settings pane has enough usable
  width
• successful connect/remove/config reload actions are reflected by card state instead of redundant
  implementation-detail notices
• saving a custom OAuth server treats its initial authorization-required reload result as expected,
  while unrelated or genuine connection failures remain visible
• failures, restart requirements, and explicit Test results remain visible

Security and compatibility

• keeps MCP OAuth separate from provider OAuth because their lifecycle and storage contracts differ
• uses PKCE, one-use state, bounded five-minute gateway flows, and an exact callback path
• validates pasted callbacks against the active flow's exact scheme, authority, path, and state before
  consuming the one-use state
• sends a pasted callback through an authenticated WebSocket mutation payload rather than a URL query
  or unauthenticated HTTP mutation
• validates configured URLs and every OAuth discovery, registration, token, refresh, and redirect
  request with the existing SSRF guard and pinned DNS transport
• stores credentials atomically outside config, scoped by server name and URL fingerprint, and never
  returns tokens in WebUI payloads
• avoids browser/discovery side effects during background startup
• preserves existing bearer-header/PAT MCP configs; OAuth inference only applies to exact known URLs
  without explicit headers

Verification

• 153 passed — MCP OAuth, MCP tool, preset, settings route, and WebSocket HTTP regression tests
• 986 passed — complete WebUI test suite
• basedpyright — 0 errors, 0 warnings, 0 notes
• ruff check and bun run lint — passed
• bun run build — passed
• isolated real gateway/browser checks verified custom None/OAuth/Headers setup, a real save and refresh,
  suppression of the expected pre-authorization reload failure, and no horizontal overflow at 320px
  or desktop width; browser console remained at 0 errors and 0 warnings
• live Xmind handoff verified popup-close cancellation; live Linear consent verified the 180x180
  nanobot logo and project website

The live smoke tests intentionally stopped before third-party account consent. Callback exchange,
persistence, reconnect, expiry, cancellation, and hot-reload paths are covered by deterministic tests.

Closes #5297

[chengyongru]

ux待优化, 但是这个pr已经不适合再扩张了