Remove orphaned diff2html viewer assets (~1MB unreferenced since #221)

[gregoryfoster]

Summary

The dashboard still ships the diff2html viewer's client assets and CSS overrides, but nothing mounts or references them. #221 stripped the notification-side diff surface; the viewer's front-end residue was left behind.

Found during the 2026-08-19 backlog staleness pass, while evaluating #126.

The orphans

Asset	Size	Referenced by
src/dashboard/static/js/vendor/diff2html-ui.min.js	1004K	nothing
src/dashboard/static/js/diff-viewer.js	4.0K	nothing
src/dashboard/static/css/input.css diff2html override block (~lines 254–280)	—	selectors that never render

Verified absent across the whole dashboard:

• No /changes/{id} route anywhere in src/dashboard/routes/.
• No template contains diff-mount, data-unified-diff, or diff-content.
• No template loads diff-viewer.js.
• Diff2HtmlUI.draw() therefore never runs.

The CSS block compiles into the shipped output.css, so the dead selectors are served to every page.

Why remove rather than keep for #222

#222 will eventually restore a diff surface, so the tempting move is to leave these in place. Two reasons not to:

1. Restore change detail (unified diff + chunk semantics) once Replicator provides a persistent content store #222 already uses a git restore anchor for exactly this. It names 89d64eb (the commit preceding the Notification Template UI/UX polish: event labels, Context layout, and strip the dead diff/significance surface #221 strip) and instructs restoring from there rather than rewriting. Deleted assets are recoverable the same way — nothing is lost.
2. Unreferenced vendor code reads as live. A megabyte of vendored JS and a block of CSS overrides targeting .diff-mount will look load-bearing to the next person working on dashboard assets. That ambiguity is the actual cost, and it compounds the longer it sits.

The pinned version is recorded in diff-viewer.js's own header comment (diff2html-ui 3.4.52), so a future restore knows what to re-vendor even without the file.

Scope

• Delete src/dashboard/static/js/vendor/diff2html-ui.min.js.
• Delete src/dashboard/static/js/diff-viewer.js.
• Remove the diff2html block from src/dashboard/static/css/input.css — the position: relative row anchor, the .d2h-tag suppression, and the preflight padding/border re-assertion added in Diff Raw Content: line numbers overlay content cells #120.
• Rebuild CSS: bash scripts/build-css.sh, then restart per AGENTS.md.
• Check docs/UI.md and docs/STYLE.md for references to the viewer or its component classes; §10 lists vendored JS explicitly (htmx, app, dark-mode, htmx-a11y) — confirm it does not also name these.
• docs/STYLE.md §11 (Overriding Vendored CSS) uses diff2html as its worked example of the @layer vendor pattern. Keep the section — the pattern is still the house rule — but it will need a different example, or a note that the example is historical.

Restore pointer

Record in the commit message that the assets are recoverable from 89d64eb (or any commit before this one), so #222 does not have to rediscover it.

Related

• Diff viewer: copyable line-anchor links #126 — line-anchor links, blocked on the same missing surface
• Content-type-aware diff: tables (CSV/Excel) and PDF #117 — content-type-aware diff, same chain
• Restore change detail (unified diff + chunk semantics) once Replicator provides a persistent content store #222 — the issue that restores the capability