feat(documents): virtualized month timeline and document cards

The timeline windows a flat item list (month labels + chunked card rows)
with @tanstack/react-virtual over the shell's scroll container — single
scroll owner, bounded DOM at four-digit corpus sizes. Columns are
measured by a ResizeObserver so the row model and the painted grid never
disagree. Cards are compact-density: kind glyph, foreground title, one
muted meta line, condition pills, a download-only badge for attachment-
class formats, and a hover/focus selection checkbox above the whole-card
click target. In-flight and failed uploads share the same footprint with
a live progress ring and translated failure copy.

Plan-Task: W2-T2, W2-T4 (upload cards)

package.json

@@ -64,6 +64,7 @@
     "@simplewebauthn/browser": "^13.3.0",
     "@simplewebauthn/server": "^13.3.1",
     "@tanstack/react-query": "5.101.2",
+    "@tanstack/react-virtual": "^3.14.5",
     "@types/sax": "^1.2.7",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",