sync: plugin UI, desktop bridge, and chat launchers

[Luhaozhu]

Community edition sync from the development tree.

Plugin-declared UI contributions

A plugin can now describe its entire front end in plugin.json, under the
extensions["org.hugagent"].ui namespace, and the host renders straight from
that declaration. The host render path no longer branches on any plugin or
tool name, so disabling or uninstalling a plugin removes its interface on the
next refresh with no host change.

• View library — src/frontend/src/plugin-ui/ ships 19 views in three
  groups (document, analytic, container/interactive), one file per view behind
  a single registry.ts extension point, with a README on adding more.
• Declarations are data, not code — field mapping goes through a
  whitelisted pointer evaluator that rejects subscripts, expressions and
  statement injection, and reads own properties only.
• Fail-soft parsing — a malformed contribution is dropped on its own and
  reported in the import result instead of blocking the install; a ui.version
  newer than the host supports is ignored as a whole.
• Data proxy — credentials are interpolated on the server and the browser
  only ever holds a source id. Hosts configured by an administrator are
  allowed, while all other upstreams must use public HTTPS.
• Plugin-supplied front-end modules — served from the plugin package with
  connect-src 'none' and path traversal rejected, rendered in an iframe
  sandboxed as allow-scripts without allow-same-origin.

Desktop capability bridge

Bridge injection moved into the catalog resolver, the single source of truth
for resolving enabled capabilities, so scheduled jobs, batch sub-agents, job
wake-ups and plan mode all use the same assembly path. Cloud gateway
configuration became an explicit bridge_servers argument with a documented
override order. Per-user effective configuration is cached for 30 seconds, and
the gateway route no longer holds a database connection while forwarding a
stream.

Chat launchers

• Typing @ now opens a structured launcher for available files, agents, plan,
  batch, workflow, and loop actions, with direct agent filtering in the second
  level.
• Typing / groups modes, plugins, and skills and shows descriptions to make
  command discovery easier.
• Keyboard handling now ignores Enter and Tab when no candidate is available,
  and entering the agent picker clears the launcher query without removing the
  mention marker.
• The English and Chinese chat guides document both launchers and their
  keyboard controls.

Fixes and hardening

• UI contributions are back-filled at startup for already-installed built-in
  plugins, so an upgrade does not leave the declared views missing.
• Contribution resolution is batched, authorization is checked per slug, the
  data proxy uses a shared HTTP connection pool, and registry lookups are
  memoized.
• Shared plugin UI strings are registered in the common dictionary.

Checks

scripts/build_ce.py ran on the final clean commit with import, release, and
front-end self-checks enabled. All gates passed: brand gate (0 text hits and a
full path scan), required-runtime-file gate, forbidden-artifact gate, binary
asset allowlist, LICENSE gate, backend import/OpenAPI/ORM checks, CE release
regression tests, and the front-end i18n, dark-mode, TypeScript, and Vite build
checks.