feat(mcp): rename to Connectors, prettier tool calls, observability

Polish on top of the initial MCP support commit. Same scope (no auth/marketplace yet),
just smoothing the rough edges from a real test session.

UX
- Settings tab + chat-input button renamed to "Connectors". MCP is mentioned in
  the page description (with a link to modelcontextprotocol.io) so the protocol
  is still discoverable.
- New `Connectors` button next to Documents / Workflows in the chat input opens a
  popover with a per-server toggle switch. Hides itself when the user has no
  connectors configured.
- Tool calls in chat now render `Running <Server> · <tool>` (friendly) instead of
  the raw `mcp__<slug>__<tool>` prefix; the original name still routes correctly.
- After each MCP tool call, a result block shows ✓/✗ + first line of output, with
  a "Show details" toggle that expands pretty-printed JSON arguments and the full
  text output.
- New connectors auto-discover their tool list immediately on save (no extra Test
  click). Re-enabling a disabled connector also auto-tests.
- Settings card redesigned: status pill, header chips, expandable per-tool
  descriptions with More/Less. Sanitises Name field if it looks like a Bearer
  token was pasted into it (best-effort safety net).
- Amber "only add connectors you trust" notice at the top of the page and a
  compact restated form inside the Add panel.

Backend
- New SSE event type `mcp_tool_result` with `{ server, tool, ok, args, output }`.
  args/output capped at 4 KB each before persistence (the model still receives
  the untruncated tool output - only the user-visible preview is capped).
- `tool_call_start` now optionally carries `display_name`; the renderer
  prefers it.