webclient: make session loads cheap, cancellable and honest ...

Opening a session moved ~1 MB of history JSON and a 1.5 MB bundle
uncompressed, then reloaded the whole transcript on every WebSocket
reconnect; a failed request wiped what was on screen.

Server: GZipMiddleware(1 KB) - one history page shrinks 5.1x, the SPA
bundle 3.2x.

Client:
- session_sync asks /meta instead of reloading history, and only when
  the view is idle;
- session loading keeps the previous transcript until the new data
  arrives, aborts the request it replaced, ignores stale responses and
  surfaces a retryable error banner;
- search-jump pagination no longer pages through the previous session's
  history;
- REST calls get a 30 s timeout and caller abort support;
- WS queue overflow rejects the message and marks the bubble "not sent"
  instead of silently dropping the oldest;
- tool card bodies mount on first open (a 38-card session drops ~1 MB of
  markup and ~25k elements);
- identical /sessions requests coalesce, the streaming watch is a cheap
  signature, the content-tool key is one store computed, useTime shares
  one 30 s ticker, connect() clears the retry timer.

e2e opens result cards explicitly now that bodies are lazy.

e2e: drive the real agent and file tools through the built webclient ...

New e2e/ suite (outside testpaths, so the normal run never collects it):
a throwaway Postgres database, a real navi-server with an isolated temp
CWD/workspace, the built SPA served by the backend and the production LLM
route from llm.json. Covers filesystem_write write/read, filesystem_write
edit plus filesystem_read diff, and file_ai edit, asserting both the
rendered tool cards and the files on disk.

Each run drops its database; MCP servers stay off because the temp CWD has
no mcp_servers.d, and failures save a screenshot, the DOM and the browser
log. docs/e2e.md documents the harness, requirements and knobs.