feat: delta/instant upload + frontend UI/UX polish

Bundles the backend+frontend delta-upload (content-dedup) feature with a
batch of frontend fixes from this session.

Upload / dedup:
- Client-hashed delta & instant upload (deltaUpload, hashWasm vendor shim)
- Backend dedup batch endpoint (dedup_service, dedup_handler, routes)
- session store owned-hash helpers; unit tests + upload-strategy bench

Frontend UI/UX:
- Colour file-type icons in grid/list (per-type tinted tiles + glyph hue)
- Robust thumbnail fallback; PDFs now show their type icon (backend
  generates no PDF thumbnails) instead of a blank tile
- Fix PDF preview: load via a same-origin blob: iframe — the API URL is
  blocked by the global X-Frame-Options: DENY in the browser's framed
  PDF viewer, matching the existing CSP `frame-src blob:` design
- Groups: localized virtual-group description (no DB schema-note leak),
  add nav.groups to the 15 missing locales, fix primary-button contrast
- Repoint --color-text-light → --color-on-accent (was faint grey on accent)
- Nudge the admin role badge off the user-menu header divider

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

frontend/src/lib/utils/display.ts

@@ -12,6 +12,50 @@ export function iconNameFromClass(iconClass: string | undefined | null): string
 	return token ? token.slice(3) : 'file';
 }
 
+/**
+ * Coarse colour bucket for a resolved icon name (see {@link iconNameFromClass}).
+ * One hue per broad file family so the grid/list glyphs render in type-specific
+ * colours instead of a flat monochrome. Each bucket is backed by a
+ * `--file-kind-*` token (variables.css) and consumed via the `.file-icon--*`
+ * modifier classes (resourceList.css).
+ */
+export function fileIconKind(iconName: string): string {
+	switch (iconName) {
+		case 'folder':
+		case 'folder-open':
+			return 'folder';
+		case 'file-pdf':
+			return 'pdf';
+		case 'file-word':
+			return 'doc';
+		case 'file-excel':
+			return 'sheet';
+		case 'file-powerpoint':
+			return 'slides';
+		case 'file-archive':
+		case 'file-zipper':
+			return 'archive';
+		case 'file-code':
+			return 'code';
+		case 'file-image':
+			return 'image';
+		case 'file-video':
+			return 'video';
+		case 'file-audio':
+			return 'audio';
+		case 'file-alt':
+		case 'file-lines':
+			return 'text';
+		default:
+			return 'generic';
+	}
+}
+
+/** `.file-icon` colour-bucket modifier class for a resolved icon name. */
+export function fileIconKindClass(iconName: string): string {
+	return `file-icon--${fileIconKind(iconName)}`;
+}
+
 /** Format a timestamp (epoch seconds/ms or ISO-8601 string) as a local date. */
 export function formatDate(value: number | string | null | undefined): string {
 	if (value === null || value === undefined) return '';