feat: add controlled browser preview workflow

[nikhilachale]

Summary

• keep Browser closed by default and treat URLs printed in terminal output as signal-only, showing the Browser indicator without navigating or stealing focus
• preserve explicit terminal-link navigation while safely serializing and coalescing preview requests
• open web links in AO Browser only for active workers; standalone shell, orchestrator, and terminated-session links continue to use the system browser
• extend the terminal-link context menu with Open in system browser and Copy link actions
• refresh explicitly selected static HTML, Markdown, and related assets only after worker activity becomes idle, waits for input, or exits
• never choose a preview target for a fresh worker; selection remains owned by ao preview, a managed preview server, or deliberate user navigation
• avoid replayed terminal output opening live previews
• document the rendered-preview workflow for framework projects: use the exact session worktree, start or reuse its development server, wait for a healthy URL, and run ao preview <url> near the end of UI work
• add an explicit Browser toolbar action for opening the current page externally
• merge current main and resolve the preview poller and Browser panel conflicts

Review fixes

• guard every poller target mutation and refresh with a URL + target-revision compare-and-set so a stale session snapshot cannot overwrite an explicit user selection
• persist the exact revision produced by a poller-owned missing-file clear, and restore only while the blank target still has that revision
• separate explicit target revisions from static-content refresh revisions; automatic refreshes now reload the Browser's current location instead of navigating back to the selected entrypoint
• replace the per-session 250 ms recursive asset walk with a complete, uncapped fingerprint at a five-second cadence while retaining cheap entry metadata checks on each poll
• preserve current main's explicit-preview ownership instead of reintroducing automatic entrypoint discovery
• restore system-browser fallback for terminals that do not have an AO Browser inspector
• route the Browser toolbar's external action through the shared error-handling policy
• remove a stale Browser layout wrapper left over from the retired viewport selector

Tests

• npm run lint (full backend tests + golangci-lint)
• cd backend && go test -race ./internal/preview ./internal/service/session ./internal/storage/sqlite/store
• cd frontend && npm test
• cd frontend && npm run typecheck
• regenerated and verified sqlc, OpenAPI, and frontend API artifacts with the repository-pinned generators

Notes

• framework applications still require their project development server; worker guidance selects the relevant route and invokes ao preview
• static file previews are daemon-driven after explicit selection and refreshed only when active worker activity finishes

[whoisasx]

Requesting changes for four correctness and scalability issues in preview ownership and refresh handling. The controlled terminal-link workflow, fresh-worker behavior, external-open support, and prompt/documentation changes otherwise look coherent. Focused backend/frontend tests, race tests, typecheck, and CI pass, but focused regression probes reproduced the ownership failures described inline.