feat(web): scope the page to the daemon its address names

[futrime]

Unit 6 of the multi-daemon plan (C6).

• Addresses name the daemon first: #/@<name>/<view>…. An address without @ is Local, so existing addresses keep working.
• api.js exports daemon(), scoped(path), hash(path) (a page address that keeps the @name), onDaemon(fn) and toDaemon(name, path).
• get, post, send and poll scope every call to /d/<name>/api/…, Local included, so Local is /d/local/api/…. /api/daemons* and /api/notifications* stay unscoped.
• Switching daemon:
  • fires hmz:daemon
  • clears the poll caches and drops answers that were still in flight for the old daemon
  • closes the held SSE streams
  • remounts the view and dialog
  • resets the sidebar lists through the event (footer untouched)
  • resets the module caches: workspaces, backends, ssh hosts
• go(), stays() and every #/ link or location.hash set in a static module go through hash(). Export links and the held stream go through scoped().
• Server: site.py answers /d/local/api/… as /api/…. Proxying other daemons is unit 4.
• Docs: addresses in docs/reference/web.md and docs/user/web.md.

Tests: new tests/integration/test_web_scoping.py (Chromium + WebKit) covers:

• #/@local/… views load
• every request goes under /d/local/api/
• links keep @local
• an epic started on @local stays there, and its stream is scoped
• switching to a routed @other fires hmz:daemon, and its requests go to /d/other/api/

All test_web_* pass (139), and unit tests pass (5690). Live hmz web check with Playwright: all requests went to /d/local/api/, with no console errors.

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[futrime]

Landed together in #385.