Let source and diff cards grow to their full height

Why

The code browser’s source card was capped at 76vh (and at calc(100vh - 240px) in the pane), so a long file scrolled inside the card. A diff card was not capped, but overflow: hidden still made it a scroll container. Pressing Ctrl+F for a symbol then found a match inside an inner box: the page stayed where it was, only the card moved, and the result was easy to lose.

What

server/web/static/vole.css only:

• .blob loses its height cap, including the .code-main .blob override and the mobile reset. The source card grows to the file’s full height. It keeps overflow-x: auto, so a line wider than the pane still scrolls sideways instead of widening the page.
• .diff now uses overflow: clip instead of overflow: hidden. It still clips its rounded corners, but it is no longer a scroll container, so find has nothing inside the card to scroll.
• The file and changed-file trees keep their capped sticky scroll. They are navigation beside the pane, not the rendered source or diff; a full-height tree would push the content off the page on a broad change.

The commit view, compare view and a change’s Files tab all render the same .diff component, so they change together.

Checks

• vole workflow run verify — all steps pass, including go test -race ./..., vet, build, licenses, links and packaging.
• ./vole workflow check — both workflows validate.

There is no new test: the change is CSS-only, and the existing suite has no browser to assert a computed height. #144’s per-file review is the behaviour this makes pleasant; opening one large diff now scrolls the page rather than a box inside it.

Closes #145.