Show sync progress in a running toast through push, pull, and preview

[shaunandrews]

Related issues

• STU-2162
• Stacked on Publish through the redesigned connect-site picker, shared with onboarding #4449 (→ Bring back preview links as a Share dialog in the header #4448 → Collapse push and pull into one Sync action with the designed dialog #4447 → Replace the site dropdown with a permanent header #4445 → Agentic UI: Port selective sync #4377). Final PR of the site-header stack.

⚠️ Visual change: needs human review in light + dark mode.

How AI was used in this PR

Claude Code built and iterated on this live in the running app (including tracking down a Notice-hook crash); verified with typecheck and unit tests.

Proposed Changes

A push, pull, or preview can run for a while, and the header only has room to spin. This adds a running toast that opens when the sync starts, updates in place as it progresses, and is replaced in place by the result — one message for the whole operation, no pile-up.

• Push shows "Uploading… X%" from the upload byte-fraction Agentic UI: Port selective sync #4377 already emits (sync-upload-progress) — the push connector now surfaces it via an onProgress callback, mirroring pull. No main-process changes.
• Pull shows the CLI's narration + percentage.
• Preview shows a publishing → published toast.

Progress is throttled (~3/sec) so the aria-live announcer isn't re-rendered dozens of times a second, and the toast store treats a non-positive duration as "persist until replaced." Also folds in the fix for a crash when a toast is replaced in place with a different shape (a running toast losing its description at the result) — the Notice is keyed on its shape so it re-mounts cleanly. (That's the same fix as the trunk-side #4443, which this #4377-based stack doesn't have.)

Testing Instructions

1. Open a connected site → Sync → Push. Confirm a single "Pushing to live → Uploading… X%" toast that resolves in place to "Push complete" — no flicker, no crash.
2. Pull → confirm the running toast shows the CLI progress and resolves to "Pull complete".
3. Share → New preview → confirm "Publishing preview link" → "Preview link published".
4. Both light and dark.

Pre-merge Checklist

• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.
• I have added or updated tests to cover my changes, if applicable.
• I have tested my changes in both light and dark mode.