General Improvements: Harden storage, anchoring, imports, and a11y; gate deploys on tests

[akashgoswami]

Correctness & data safety:

• Persist deletions immediately (before the undo toast expires) so a reload in the undo window can no longer resurrect a deleted comment; Undo restores idempotently and only into the page it belongs to.
• Surface storage failures: store.set reports success, an explicit banner/toast warns when comments cannot be persisted, and unsaved comments stay in memory and remain exportable.
• Single schema gate for storage/import: reject unknown geometry kinds and comment types, id-less records, malformed anchors/replies, and oversized batches; dedupe within-batch and existing-project IDs.
• SPA isolation: a non-explicit page key tracks location.pathname and recomputes only on refresh(), so routes keep separate comments.

Anchoring:

• Refuse to guess: highlight quotes that are lost or ambiguous (even accounting for prefix/suffix context) surface an explicit unanchored pill instead of highlighting an arbitrary copy.
• Shapes/pins whose anchor element is gone show a distinct "anchor missing" badge instead of silently redrawing over .
• Coalesced ResizeObserver/scroll relayout re-renders only when an anchor actually moved.

Accessibility:

• Markers are focusable role=button with aria-labels and open on Enter/Space; add :focus-visible outlines.

Tooling:

• Add Firefox/WebKit Playwright matrix workflow and gate Pages deploy on the chromium suite; switch demo server to serve on port 4200.
• Ignore .env and opencode.json.