[codex] Use progressbar for autosave progress

[ashfame]

What changed

This is the semantic accessibility fix for #3815. The in-progress save/autosave indicator no longer uses role="status" as the progress surface. Instead, the indicator is exposed as a role="progressbar" with exact aria-valuenow and aria-valuetext values.

A separate visually hidden polite status region now announces only meaningful transitions, such as save/autosave completion and failure. The visible progress ring still updates with exact progress.

The existing Playwright autosave status sampling was updated to include progressbar nodes, assert exact progress on aria-valuetext, and assert that percent progress is no longer exposed through role="status".

Approach

The in-progress indicator now uses:

• role="progressbar"
• aria-label="Autosave progress" or aria-label="Save progress"
• aria-valuemin="0"
• aria-valuemax="100"
• aria-valuenow={progressPercent}
• aria-valuetext, for example Autosaving 42%

A hidden role="status" live region announces only completion/failure messages and skips initial render announcements.

Pros

• Models progress with the correct ARIA role.
• Lets screen reader users inspect the exact current percentage without unsolicited percent-by-percent announcements.
• Keeps visual progress fidelity unchanged.
• Separates queryable progress state from live status announcements.
• Avoids stale bucketed labels.

Cons

• Larger change than simple bucketing.
• Adds state/effect logic for transition announcements.
• Screen readers generally will not announce progressbar value changes unless users navigate to the progressbar, so completion/failure needs the separate live region.
• Slightly more test maintenance because autosave sampling must include progressbar nodes.

Validation

• npm exec nx -- run playground-website:typecheck
• npm exec nx -- run playground-website:lint