diffr web: draw comparisons as plain unified rows on phones

[milanb17]

Summary

On phones, draw the comparison as plain unified rows instead of Monaco's diff editor, keeping diffr's folds and summaries. Stacks on #1093; merge it first.

viewport ≤ mobile breakpoint → MobileDiffView (DOM rows, built near the viewport)
otherwise                    → Monaco diff editor, unchanged

• reviewUnifiedRows.ts turns a file's structural diff into the rows a reader sees: lines, and the bands diffr's collapsed folds leave in their place, with pseudocode summaries as the band's detail. It reads the same rows, gaps and foldables the diff editor uses.
• Folding uses large targets: the whole header row, and the whole band, toggle a fold. Only folds spanning two or more lines get a chevron, since folding one line saves nothing and every chevron turns its row into a tap target. A tap that ends a text selection doesn't fold.
• Pseudocode wraps at phone width. Syntax colours come from reviewSyntaxRoles.ts, extracted from reviewStructuralSyntax.ts so both views share them.
• Find highlights matches with CSS Custom Highlights on phones. Crossing the breakpoint (rotation, resizing) re-routes to the other view.
• Not ported yet: pinch to fold, per-scope viewed checkboxes, recursive fold, open on GitHub from the file header, and lens section headers.

Evidence

• iPhone 17 Pro simulator, vite v5.3.0...v5.4.0 (217 files), cold loads, three runs each:

	Peak	Settled
Monaco	724–1031 MB	435–445 MB
Unified rows	696–759 MB	374–422 MB

• A separate simulator pass checked folding and unfolding, pseudocode bands, file headers, the file tree, theme switching and rotation. Monaco's "potential listener LEAK" warnings (200/300/400 listeners) are gone on phones.
• Known: one transient blank viewport after about 60 rapid flings, not yet reproduced; dragging selection handles is unverified because the simulator can't do it; bands can shift briefly during sideways swipes.
• Browser typecheck, lint and format pass.

Merge Danger

Door: two-way; revert the commit.

Blast Radius: phone-width viewports in diffr web only; desktop still uses Monaco. Phone users lose the features listed as not ported.