feat(downloads): share UI — user picker + public watch links + watch page

Rework the share dialog into two clear modes and add the public /watch player page:
- ShareDialog: (A) 'Share with a user' — autocomplete picker over registered users (was a blind
  email box that 404'd on non-users); (B) 'Share a link' — create/list/copy/revoke public links
  with allow-download toggle, optional expiry (1/7/30d), optional password; per-link view count.
- WatchPage: standalone login-free player at /watch/<token> (routed in main.tsx like /privacy),
  self-contained mini-i18n (en/hu/de by browser language); password gate → unlock → play; shows a
  Download button only when the link allows it.
- api: ShareLink/ShareRecipient types + link CRUD + recipients; share i18n (en/hu/de).
Verified end-to-end in a real browser: user picker, link create, public playback, stream-only vs
downloadable, password gate + unlock, no console errors.

frontend/src/i18n/locales/de/downloads.json

@@ -81,7 +81,30 @@
     "label": "E-Mail des Empfängers",
     "placeholder": "user@example.com",
     "submit": "Teilen",
-    "done": "Geteilt mit {{email}}"
+    "done": "Geteilt mit {{email}}",
+    "failed": "Teilen fehlgeschlagen.",
+    "userSection": "Mit einem Nutzer teilen",
+    "userHint": "Erscheint bei ihm unter „Mit mir geteilt”.",
+    "pickPlaceholder": "Nutzer suchen…",
+    "noUsers": "Keine weiteren Nutzer.",
+    "linkSection": "Per Link teilen",
+    "linkHint": "Jeder mit dem Link kann zusehen — ohne Konto.",
+    "noLinks": "Noch keine Links.",
+    "newLink": "Link erstellen",
+    "createLink": "Erstellen",
+    "allowDownload": "Download erlauben",
+    "expiry": "Läuft ab",
+    "expiryNever": "Nie",
+    "days": "{{n}} Tage",
+    "password": "Passwort (optional)",
+    "copy": "Link kopieren",
+    "copyFailed": "Kopieren fehlgeschlagen.",
+    "revoke": "Widerrufen",
+    "revokeConfirm": "Der Link funktioniert sofort nicht mehr.",
+    "views": "{{n}} Aufrufe",
+    "expiresOn": "läuft ab {{date}}",
+    "hasPassword": "Passwort",
+    "downloadable": "herunterladbar"
   },
   "confirm": {
     "cancelTitle": "Download abbrechen?",