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/static/locales/ar.json

@@ -52,7 +52,8 @@
     "trash": "سلة المهملات",
     "sharedwithme": "مشتركة معي",
     "profile": "الملف الشخصي",
-    "shared_with_me": "مشتركة معي"
+    "shared_with_me": "مشتركة معي",
+    "groups": "المجموعات"
   },
   "photos": {
     "empty_state": "لا توجد صور بعد",

frontend/static/locales/de.json

@@ -52,7 +52,8 @@
     "trash": "Papierkorb",
     "sharedwithme": "Mit mir geteilt",
     "profile": "Profil",
-    "shared_with_me": "Mit mir geteilt"
+    "shared_with_me": "Mit mir geteilt",
+    "groups": "Gruppen"
   },
   "photos": {
     "empty_state": "Noch keine Fotos",

frontend/static/locales/es.json

@@ -52,7 +52,8 @@
     "trash": "Papelera",
     "sharedwithme": "Compartidos conmigo",
     "profile": "Perfil",
-    "shared_with_me": "Compartidos conmigo"
+    "shared_with_me": "Compartidos conmigo",
+    "groups": "Grupos"
   },
   "photos": {
     "empty_state": "Aún no hay fotos",
@@ -365,7 +366,8 @@
     "new_folder": "Nueva carpeta",
     "share": "Compartir",
     "view": "Ver",
-    "selected_count": "{{count}} seleccionados"
+    "selected_count": "{{count}} seleccionados",
+    "uploaded_saved": "Subida completa — {{mb}} MB deduplicados"
   },
   "dialogs": {
     "rename_folder": "Renombrar carpeta",

frontend/static/locales/fa.json

@@ -52,7 +52,8 @@
     "trash": "سطل زباله",
     "sharedwithme": "به اشتراک‌گذاشته شده با من",
     "profile": "پروفایل",
-    "shared_with_me": "به اشتراک‌گذاشته شده با من"
+    "shared_with_me": "به اشتراک‌گذاشته شده با من",
+    "groups": "گروه‌ها"
   },
   "photos": {
     "empty_state": "هنوز عکسی نیست",

frontend/static/locales/fr.json

@@ -52,7 +52,8 @@
     "trash": "Corbeille",
     "sharedwithme": "Partages avec moi",
     "profile": "Profil",
-    "shared_with_me": "Partages avec moi"
+    "shared_with_me": "Partages avec moi",
+    "groups": "Groupes"
   },
   "photos": {
     "empty_state": "Pas encore de photos",

frontend/static/locales/hi.json

@@ -52,7 +52,8 @@
     "trash": "रद्दी",
     "sharedwithme": "मेरे साथ साझा किए गए",
     "profile": "प्रोफ़ाइल",
-    "shared_with_me": "मेरे साथ साझा किए गए"
+    "shared_with_me": "मेरे साथ साझा किए गए",
+    "groups": "समूह"
   },
   "photos": {
     "empty_state": "अभी कोई फ़ोटो नहीं",

frontend/static/locales/it.json

@@ -52,7 +52,8 @@
     "trash": "Cestino",
     "sharedwithme": "Condivisi con me",
     "profile": "Profilo",
-    "shared_with_me": "Condivisi con me"
+    "shared_with_me": "Condivisi con me",
+    "groups": "Gruppi"
   },
   "photos": {
     "empty_state": "Nessuna foto ancora",

frontend/static/locales/ja.json

@@ -52,7 +52,8 @@
     "trash": "ゴミ箱",
     "sharedwithme": "自分と共有",
     "profile": "プロフィール",
-    "shared_with_me": "自分と共有"
+    "shared_with_me": "自分と共有",
+    "groups": "グループ"
   },
   "photos": {
     "empty_state": "写真はまだありません",

frontend/static/locales/ko.json

@@ -52,7 +52,8 @@
     "trash": "휴지통",
     "sharedwithme": "나와 공유됨",
     "profile": "프로필",
-    "shared_with_me": "나와 공유됨"
+    "shared_with_me": "나와 공유됨",
+    "groups": "그룹"
   },
   "photos": {
     "empty_state": "아직 사진이 없습니다",

frontend/static/locales/nl.json

@@ -52,7 +52,8 @@
     "trash": "Prullenbak",
     "sharedwithme": "Gedeeld met mij",
     "profile": "Profiel",
-    "shared_with_me": "Gedeeld met mij"
+    "shared_with_me": "Gedeeld met mij",
+    "groups": "Groepen"
   },
   "photos": {
     "empty_state": "Nog geen foto's",

frontend/static/locales/pl.json

@@ -52,7 +52,8 @@
     "trash": "Kosz",
     "sharedwithme": "Udostępnione dla mnie",
     "profile": "Profil",
-    "shared_with_me": "Udostępnione dla mnie"
+    "shared_with_me": "Udostępnione dla mnie",
+    "groups": "Grupy"
   },
   "photos": {
     "empty_state": "Brak zdjęć",

frontend/static/locales/pt.json

@@ -52,7 +52,8 @@
     "trash": "Lixeira",
     "sharedwithme": "Compartilhados comigo",
     "profile": "Perfil",
-    "shared_with_me": "Compartilhados comigo"
+    "shared_with_me": "Compartilhados comigo",
+    "groups": "Grupos"
   },
   "photos": {
     "empty_state": "Nenhuma foto ainda",

frontend/static/locales/ru.json

@@ -52,7 +52,8 @@
     "trash": "Корзина",
     "sharedwithme": "Доступно мне",
     "profile": "Профиль",
-    "shared_with_me": "Доступно мне"
+    "shared_with_me": "Доступно мне",
+    "groups": "Группы"
   },
   "photos": {
     "empty_state": "Фотографий пока нет",

frontend/static/locales/zh-TW.json

@@ -52,7 +52,8 @@
     "trash": "回收站",
     "sharedwithme": "與我共享",
     "profile": "個人資料",
-    "shared_with_me": "與我共享"
+    "shared_with_me": "與我共享",
+    "groups": "群組"
   },
   "photos": {
     "empty_state": "還沒有照片",

frontend/static/locales/zh.json

@@ -52,7 +52,8 @@
     "trash": "回收站",
     "sharedwithme": "与我共享",
     "profile": "个人资料",
-    "shared_with_me": "与我共享"
+    "shared_with_me": "与我共享",
+    "groups": "群组"
   },
   "photos": {
     "empty_state": "还没有照片",