feat(pixel): prepare local drafts while the runtime is unavailable

[0xacee]

Why this matters

Pixel currently disables its text composer while service health is connecting, unreachable, or switching models. Users cannot prepare a local message during that wait even though drafting and persistence require no model request.

Separate the active-task editing lock from runtime readiness. Text entry, local prompt commands and confirmed text-file insertion stay available while the backend is unavailable. Send and dictation retain their readiness gate. The existing sendMessage guard also rejects Enter while unavailable; readiness returning never submits automatically.

The contract is: edit locally → existing draft autosave → reload restores the same draft → wait for an available runtime → explicitly send through the existing authenticated stream. Sending, stopping, and restored active/checking tasks still lock editing. No storage schema or request format changes; reverting restores the earlier UI gate and leaves saved drafts readable.

Regression evidence and validation

• Five of six new mounted-page tests fail on the base. Fixed cases cover connecting/unreachable/switching, local prompt insertion, readiness recovery without auto-send, and restored active-task locking. Six focused files: 119 tests pass.
• node scripts/check-unavailable-draft.mjs: actual Chromium 153 against the production page with controlled API routes. The base disables editing in both unavailable modes. The fix preserves a Vietnamese/Unicode draft across reload, sends nothing on Enter or readiness recovery, and sends exactly once with the original content after clicking Send.
• npm run build passes. Focused ESLint has zero errors and 39 existing JSX warnings.
• Applied the exact fix(pixel): restore the authoritative legacy conversation before autosave #4970, fix(history): restore keyboard focus after confirmed chat deletion #4974 and fix(pixel): preserve newer conversations from stale editors #5004 changes together with this patch: 10 files / 139 tests pass. Both Chromium scripts pass on that combined tree, preserving newer saved data, downloading unsaved recovery text, rejecting stale sends, and retaining explicit-send-only behavior during backend recovery.

The browser fixture serves the UI normally and makes the backend unreachable; it does not establish offline site loading or an installed PWA lifecycle. An installed ODS runtime and native macOS browser were not exercised. Ready for review; deployed lifecycle checks and independent review remain pending before merge.

Overlap check

Searched live open/closed PR titles and production-file changes for Pixel composer, draft, offline/unavailable, switching and persistence. #4951 changes prompt-space calculation; #4770/#4769 concern badge/rewind behavior; #4396 identity; #4255 conversation finding; #4220/#4219 context selection and scrolling; #4218/#4213 local commands. None enables local drafting during runtime unavailability.

#4970 restores legacy conversation authority, #4974 restores deletion focus, and #5004 rejects already-visible stale writes. They solve separate behaviors in shared Pixel files. Use merge order #4970 → #4974 → #5004 → this PR and retain all regressions. #4970's lock assertion was updated to query the textbox role rather than the old unavailable placeholder, without weakening the active-task requirement.