feat(app): add the snapshot reviewer

[thetaPC]

Issue URL: N/A

What is the current behavior?

Snapshot-heavy PRs are effectively unreviewable on GitHub: the Files tab won't render diffs for hundreds of binary files, and a Playwright bump or a branch sync touches thousands. Reviewers end up opening screenshots one at a time or skipping them.

What is the new behavior?

A local app that reads each screenshot's before and after straight out of git, with no checkout, and serves them for review.

• Four views per screenshot: side by side, slider, onion skin and pixel diff
• List sorted by how much changed, filterable by status, browser, mode, theme, direction and palette
• Whole directories can be checked off at once, because we only read one browser per set
• Viewed marks sync to the PR's Files tab, and comments post back to the PR as file comments
• Progress carries across pushes: a mark resets only when its screenshot changed, the same rule as the Files tab
• Flags snapshots that no test generates anymore, and changes that only re-encoded the image
• Works read-only when gh isn't signed in, and warns about it in the terminal
• A 2,057 screenshot PR opens in 1.45s, or ~14s the first time while diffs are generated

Does this introduce a breaking change?

• Yes
• No

Other information

To try it, clone this repo next to ionic-framework, run npm ci, then npm start -- --pr 31435 for a quick 51-screenshot pass, or npm start -- --pr 31465 for every case the reviewer handles (renames, removals, orphans, re-encodes, existing comments).

Both PRs are merged, so "Post comments" is disabled for them. Marking files viewed is safe.

Hosted Vercel previews, so a review doesn't need a local clone, will be looked into separately.