feat(capture): on-page notice while screenshotting

[DCCA]

Why

During full-page capture the editor activates and scrolls the target tab, so the user is looking at the page — not the editor's "Capturing N/M" progress text. If they switch tabs or scroll mid-capture, captureVisibleTab grabs the wrong frame and the stitched screenshot breaks. There was no on-page signal telling them to hold still.

What

The content script injects a fixed, top-of-viewport notice on the page being captured:

⟳ Capturing full page… Please don't switch tabs or scroll until it finishes

Lifecycle (driven by captureFullPage):

• SB_CAPTURE_BEGIN → show the notice; a 450ms pause lets the user read it.
• Per segment: SB_SCROLL_TO re-shows it, then SB_SET_OVERLAY {visible:false} hides it (display:none, painted via rAF) before captureVisibleTab so it's never baked into a frame.
• SB_RESTORE_SCROLL / SB_CAPTURE_END → remove it.

The notice is position:fixed, max z-index, pointer-events:none, self-contained inline styles (no dependence on page CSS). All overlay messages are best-effort (notify swallows errors) so a missing/old content script can never abort a capture.

Verification

• npm run typecheck, npm run lint, npm run test (54 pass, unchanged — additive DOM/messaging at the chrome.* boundary), npm run format:check, npm run build green.
• Confirmed the notice strings + SB_CAPTURE_BEGIN are in the built dist/content.js.
• Visual confirmation (notice appears during capture, absent from the saved screenshot) to be done by @DCCA after reload — no runner exists for the live extension; this is the repo's standard manual-QA path for capture UI.

🤖 Generated with Claude Code