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/en/downloads.json

@@ -81,7 +81,30 @@
     "label": "Recipient email",
     "placeholder": "user@example.com",
     "submit": "Share",
-    "done": "Shared with {{email}}"
+    "done": "Shared with {{email}}",
+    "failed": "Couldn’t share.",
+    "userSection": "Share with a user",
+    "userHint": "They’ll find it under “Shared with me”.",
+    "pickPlaceholder": "Search a user…",
+    "noUsers": "No other users yet.",
+    "linkSection": "Share a link",
+    "linkHint": "Anyone with the link can watch — no account needed.",
+    "noLinks": "No links yet.",
+    "newLink": "Create a link",
+    "createLink": "Create link",
+    "allowDownload": "Allow download",
+    "expiry": "Expires",
+    "expiryNever": "Never",
+    "days": "{{n}} days",
+    "password": "Password (optional)",
+    "copy": "Copy link",
+    "copyFailed": "Couldn’t copy.",
+    "revoke": "Revoke",
+    "revokeConfirm": "This link will stop working immediately.",
+    "views": "{{n}} views",
+    "expiresOn": "expires {{date}}",
+    "hasPassword": "password",
+    "downloadable": "downloadable"
   },
   "confirm": {
     "cancelTitle": "Cancel download?",