fix(changes): a huge line no longer cuts the diff short; preview pages up to 64 MB

A page that inlines a video as base64 has rows of megabytes. Its patch was read
at the 200 KB per-file cap, so the first such row used the whole budget and
every hunk after it was lost ("Diff cut short"). The patch is now read up to
16 M characters, rows over 4,000 characters are cut to 1,000 with an `elided`
count the diff shows, and only then is the cap applied. Hunk actions re-read
git's own diff by header and are unchanged.

The HTML preview refused a page over 10 MB and showed only the file icon; a
self-contained 18 MB slide deck now opens (limit 64 MB).

client/src/lib/types.ts

@@ -2974,6 +2974,9 @@ export interface LaneDiffLine {
   /** Line number on the right (after) side; null on a removed line. */
   newLine: number | null;
   text: string;
+  /** Characters cut off the end of a very long row (a base64 blob, a minified
+   *  bundle): `text` holds only its start. Absent on a whole row. */
+  elided?: number;
 }
 
 /** `GET /api/lanes/:id/git/blame` — Annotate: who last changed each line of