Draw diffr web's comparison as plain rows on every screen

[milanb17]

Summary

Draw diffr web's comparison as plain DOM rows on desktop as well as phones, in split or unified layout, and delete the Monaco diff editors. Stacks on #1095; merge #1093, #1094 and #1095 first. It deletes reviewFilesDiffView.ts, which #1096 edits, so #1096 can close once this merges.

• Split layout. Each row pairs the lines diffr paired, with a hatched filler where a side has no line. When lines don't wrap, each side scrolls sideways on its own, and a file's segments stay level. With word wrap on, both sides of a row wrap to the same height. A text selection stays on the side it started on.
• Unified layout. On desktop it shows both sides' line numbers. Phones keep one column of numbers. Removed header lines now carry their base-side fold pill, as head lines already did.
• Folds. Bands, chevrons (2+ lines), header pills and summary pseudocode all work as before. On desktop only the gutter folds, so clicking code just places the caret that z a/o/c/A/O/C and z j/k act on. On a phone, a tap anywhere on a fold row folds it.
• Virtualisation. Rows are drawn in blocks of 120, only near the screen. Far from it, a block empties again and holds its measured height. Before a block is drawn, its height is estimated from its row and band counts. Find, next/previous change and reveal draw the blocks they need.
• Scopes. Every open scope draws a gray indent guide down its body, broken at bands. On desktop the pointer's column picks the scope: over code it is the innermost, and moving left into the indentation steps out one guide at a time. The picked scope lights its chevron, braces and a rail (which stops at bands), shows its unread changes and a viewed box, and a click in its indentation or on the rail folds it (the header then reads "click · fold N lines"). Clicking the number of a line inside a scope lists every scope around it, innermost first, as a menu on desktop or a sheet of 56px rows on a phone. A viewed scope folds, dims its lines, keeps "Viewed" on its outermost header and shows ✓ in its pill. On a phone, pinch in over a scope to fold it, out over a fold to open it; the list sets touch-action: pan-x pan-y, so touch listeners stay passive.
• Wrapping. Wrapped lines continue under their own indentation, and their number stays beside the first line.
• Sidebar. FilesSidebar holds the changed-files tree, its summary line and a drag sash. ⌘B toggles it.
• Deleted: the multi-diff view, its structural folds, syntax decorations, the Monaco widgets for scope rails and viewed controls (redrawn as DOM above), the native-scroll shims, the overflow-widget host, and the CSS for all of these. nativeDiffScroll.ts shrinks to viewport.ts, which holds only the phone media query.

I started from our renderer rather than forking @pierre/diffs, whose line-hunk model and built-in Shiki don't fit diffr's folds. I took the idea of per-side scroll columns from its split layout but copied none of its code.

Evidence

vite v5.3.0...v5.4.0 (217 files):

	Before	After
Expand pnpm-lock.yaml (25k rows), split, desktop Chrome	one 440 ms task, 107k nodes	35 ms
Scroll through it, p95 frame	46 ms	10 ms
Same on a 402×874 touch viewport	—	40 ms expand, 11 ms p95
JS bundle (raw / gzip)	2.95 MB / 782 kB	2.11 MB / ~555 kB
iPhone 17 Pro simulator Safari, peak footprint while loading	527–543 MB	450 MB, settles at 248 MB

• Desktop Chrome: split and unified, wrapped and not, sideways sync, ⌘F highlights in undrawn blocks, [/] across blocks, the fold keys, the sidebar and quick open all checked, with no console errors. Estimated block heights matched measured ones exactly, so the page doesn't jump as blocks fill.
• Simulator Safari: 60 stacked flings showed 13 blank captures. With blocks turned off (one block per file), the same run showed 14. These are the Safari paint lag in the 82k-px CHANGELOG seen before this change, not undrawn rows.
• Scopes, desktop Chrome: hovering inside a function draws its rail in each pane segment; ticking its viewed box folds and dims it, the enclosing function then counts as viewed and carries "Viewed" and ✓, and unticking restores everything. Pinch: synthetic two-finger touch events fold and reopen a scope in phone-emulated Chrome and in iPhone 17 Pro simulator Safari (iOS 26.5), where the list's touch-action also reads pan-x pan-y. A real two-finger gesture in the simulator is still untried.
• Browser typecheck, lint and format pass. diffr-web has no unit tests.

Merge Danger

Door: two-way; revert the two commits to bring the Monaco diff view back.

Blast Radius: diffr web's whole diff view on every screen size.