feat(comments): UAT-friendly long-session QoL — drop 20-item cap, capture page URL

[AgentEnder]

Closes #319.

Summary

• Drop the 20-item count cap on comments. The hard MAX_COMMENT_ITEMS = 20 in constants.ts predates the 2 MB byte-size trim that was later added (in commit d1877670 fix) as the principled sessionStorage-quota safeguard. The count cap silently dropped older comments during longer review sessions; the byte trim already handles real storage pressure (~500–2000 typical comments fit under 2 MB).
• Capture the page URL on each comment. Multi-page review sessions had no way to tell which page a given comment came from. We now:
  • Store url?: string on CommentItem (captured from window.location.href at creation).
  • Show a third line in the comments dropdown row under the comment text — host is elided when it matches the current page, full URL is shown via title on hover.
  • Append at <url> to the copied content (single + copy-all) on its own indented line, mirroring the existing in <Component> (at <file>) convention from formatStackContext. Both the plain-text and rich-MIME (application/x-react-grab) entries get the decorated content so paste targets stay consistent.

Why

From #319: react-grab is being used for long UAT-style review sessions where reviewers comment across many pages. The 20-item silent drop and missing URL context were the two friction points called out.

Test plan

• Comment on 25+ different elements in a single session — confirm none silently disappear from the dropdown.
• Navigate to a second page, comment, open dropdown — confirm the URL line shows the new page's path under the comment.
• Comment on a cross-host page (e.g. localhost:3000 then localhost:3001) — confirm host appears in the URL label when it differs from the current page.
• Hover the URL line — confirm the full URL appears in the native tooltip.
• Use "Copy" on a single comment — confirm pasted content has at <url> on its own line.
• Use "Copy all" — confirm each entry's snippet ends with its own at <url> line.
• Verify pnpm --filter react-grab typecheck is clean.
• Run the existing comments e2e specs (packages/react-grab/e2e/clear-history-prompt.spec.ts and friends) — no regressions.

Notes

• The byte-budget trim is now the only cap. If a single user ever does hit the 2 MB ceiling (well past typical review-session sizes), an IndexedDB-backed storage layer is the natural follow-up — left out of scope here.
• No new e2e coverage for the URL display / 25+ items added in this PR; happy to add it as a follow-up if the maintainers prefer.

---

Note

Medium Risk
Changes comment persistence and clipboard output: comments are no longer capped by count and copied snippets now include a URL line, which could affect consumers expecting the previous format or increase sessionStorage churn (though still size-trimmed).

Overview
Comments history is now retained beyond the previous 20-item limit by removing the hard count cap and relying solely on the existing sessionStorage byte-size trimming.

Each CommentItem now records url (captured at creation), surfaces a compact URL label in the comments dropdown (with full URL on hover), and appends an at <url> line when copying single comments or using copy-all (including per-entry payloads).

^{Reviewed by Cursor Bugbot for commit e33a67a. Bugbot is set up for automated code reviews on this repo. Configure here.}

[vercel]

@AgentEnder is attempting to deploy a commit to the Million Team on Vercel.

A member of the Team first needs to authorize it.

[cubic-dev-ai]

No issues found across 5 files