fix(web): make the file lightbox a real scrim viewer

[gbasin]

Problem

The file preview overlay didn't feel like a lightbox: it was a solid full-screen page, documents sat left-aligned with a dead right half, the mouse wheel only scrolled when directly over the text, and previews taller than the window couldn't scroll at all on desktop.

Root causes:

• The renderer root didn't stretch inside the flex preview section, so content sized to itself and stuck left (mx-auto had no room to work) and the scroll container was only content-wide.
• md:grid-rows-none left the desktop grid row auto-sized, so tall content grew the row past the viewport instead of scrolling — the scroll container never overflowed.

Change

Drive/Slack-style scrim viewer (shared Lightbox, so it applies to /files, chat attachments, quote cards, and unfurls alike):

• Root is a dimmed, blurred scrim over the page; header controls, nav arrows, and the zoom/pager pills float on it as dark glass (fixed colors — theme tokens invert in light mode and would be unreadable on the scrim).
• Renderers stretch to the full section and mark their empty margins with data-lightbox-backdrop: the wheel scrolls from the side gutters, and a stationary click there closes the lightbox (pans, text-selection drags, and scrollbar clicks are filtered out).
• Text-like content renders as a centered paper card: markdown/notebooks at reading width, code/CSV/JSON in wider cards; images/video center fit-to-view with a shadow; PDF pages float on the scrim with a glass pager; edit/conflict modes keep a solid work surface.
• Desktop grid row fixed to minmax(0,1fr) so tall previews actually scroll.
• Filmstrip and info/history panels intentionally unchanged.

Validation

• pnpm check green; pnpm e2e 119/120 with one unrelated flake (message-action-sheet overflow popover) that passes in isolation.
• Browser-QA'd against a built preview: centering geometry, gutter wheel-scroll, scrim-click close, Esc, image zoom pill, filmstrip nav.