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/routes/search/+page.svelte

@@ -10,7 +10,7 @@
 	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';
 
 	const query = $derived(page.url.searchParams.get('q') ?? '');
 
@@ -265,7 +265,7 @@
 					onkeydown={(e) => e.key === 'Enter' && openFolder(folder)}
 				>
 					<div class="name-cell">
-						<span class="file-icon"><Icon name="folder" /></span>
+						<span class="file-icon file-icon--folder"><Icon name="folder" /></span>
 						<span>{folder.name}</span>
 					</div>
 					<div class="path-cell">{folder.path}</div>
@@ -283,7 +283,9 @@
 					onkeydown={(e) => e.key === 'Enter' && openFile(file)}
 				>
 					<div class="name-cell">
-						<span class="file-icon"><Icon name={iconNameFromClass(file.icon_class)} /></span>
+						<span class="file-icon {fileIconKindClass(iconNameFromClass(file.icon_class))}"
+							><Icon name={iconNameFromClass(file.icon_class)} /></span
+						>
 						<span>{file.name}</span>
 					</div>
 					<div class="path-cell">{file.path}</div>