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/components/ResourceList.svelte

@@ -59,7 +59,7 @@
 	import { t } from '$lib/i18n/index.svelte';
 	import { files as filesStore } from '$lib/stores/files.svelte';
 	import { formatBytes } from '$lib/utils/format';
-	import { formatDate, iconNameFromClass } from '$lib/utils/display';
+	import { formatDate, iconNameFromClass, fileIconKindClass } from '$lib/utils/display';
 	import { gridColumns } from '$lib/utils/grid';
 
 	interface Props {
@@ -301,6 +301,7 @@
 </script>
 
 {#snippet row(entry: ResourceEntry)}
+	{@const iconName = entry.kind === 'folder' ? 'folder' : iconNameFromClass(entry.iconClass)}
 	<!-- svelte-ignore a11y_no_noninteractive_tabindex -->
 	<div
 		class="file-item"
@@ -323,8 +324,8 @@
 			</div>
 		{/if}
 		<div class="name-cell">
-			<span class="file-icon">
-				<Icon name={entry.kind === 'folder' ? 'folder' : iconNameFromClass(entry.iconClass)} />
+			<span class="file-icon {fileIconKindClass(iconName)}">
+				<Icon name={iconName} />
 			</span>
 			<span class="name-cell__text">{entry.name}</span>
 		</div>