feat(dweb): user kill switch to shut down all dweb networking

[NotASithLord]

Implements the network kill switch from docs/specs/FEATURE-FIRST-CLASS-MESSAGING.md §2 — turning the dweb on is a deliberate user action, so shutting it down must be one too.

What

• New dweb/base/stop SW route (background/routes/dweb.js) — symmetric to dweb/base/start:
  • Persists dwebEnabled = false first, so it won't auto-restart on the next unlock (maybeStartBaseNetwork already gates on the setting, service-worker.js:2639).
  • Tears down a live host via the existing complete offscreen teardown (dweb/base-host/stop, offscreen/dweb-base.js:388-397: closes every room, drops the mesh + lobby, clears the re-sub timer).
  • Never spawns the offscreen just to stop it (checks getContexts first).
  • Gated on DWEB_ENABLED only (not dwebOn()) so it can stop precisely as it flips the setting off.
• "Stop the network" control in the home Network section (home/network-section.js), confirm-gated since it drops live connections.

Net effect: one user action turns all dweb networking off and keeps it off until they start it again — also the master fail-closed for the §7 unattended messaging surface once that lands.

Scope

The offscreen-side teardown already existed and is complete; this PR wires it to a user-reachable route + UI. The inbound-monitor teardown referenced in spec §2 is N/A until those monitors exist (workstream B).

Verification

bun run lint and bun run typecheck clean. (Extension UI/SW — not dev-server-previewable; reload the unpacked extension to exercise.)

🤖 Generated with Claude Code