feat(files): bookmarkable file preview URL + multi-drag count ghost (#500)

Two parity gaps from the VanillaJS → Svelte migration (issue #500):

1. URL anchor when viewing a file. Opening a file now writes `?file=<id>`
   to the URL, so a preview is bookmarkable, reload-restorable, and
   Back/Forward open/close it. The viewer is driven from the URL via two
   effects (URL→viewer with `untrack` so a user close can't be re-opened;
   viewer→URL to drop the param on close with replaceState). Replaces the
   write-less, load-only `maybeOpenDeepLink` (the `?file` reader that was
   effectively dead because nothing ever set the param).

2. Multi-selection drag ghost. Dragging more than one item now sets a custom
   drag image: a stack of the first few rows plus a count badge, reusing the
   already-ported but orphaned `.drag-preview`/`.dragged-items`/
   `.dragged-items-badge` styles. Previously a multi-item drag showed only the
   browser's default single-row ghost with no count.

Also adds `static/geo/` to .prettierignore (the bundled minified world
basemap from #499 is a data asset and must stay byte-faithful — it was
failing `prettier --check`, blocking the gate).

Frontend gate green: svelte-check 0/0, eslint, stylelint, prettier, 47 Vitest.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/.prettierignore

@@ -15,3 +15,4 @@ static/locales/
 static/vendors/
 static/workers/
 static/basemaps/
+static/geo/