Productivity features, UI translations, local email translation

- Snooze (with a Snoozed folder), keyboard shortcuts with a Settings
  section, mail templates (Compose > More > Templates), a Subscriptions
  folder with bulk unsubscribe/delete, and tracker blocking with a
  Privacy toggle (on by default)
- Own dialogs instead of browser popups; emoji picker in compose; compose
  windows survive a visit to Settings
- Sentence completion learned from the user's sent mail (no AI)
- Interface in English and Polish (one file per language, English as the
  fallback), picked from the browser on first run, then kept
- Email translation on the server via LibreTranslate: Translate button
  and "Translate to...", messages in several languages translated part
  by part, names/addresses/emoji/file names/phone numbers kept, an
  animated line while translating
- Antispam "Mark as spam" loads with the message, not when the menu opens

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/app/api/schemas.py

@@ -27,6 +27,8 @@ class ThreadSummary(BaseModel):
     has_attachments: bool
     pinned: bool
     last_message_at: dt.datetime
+    # Set while the thread is snoozed: when it comes back to the Inbox.
+    snoozed_until: dt.datetime | None = None
 
 
 class ThreadListOut(BaseModel):
@@ -64,6 +66,13 @@ class MessageEnvelope(BaseModel):
     snippet: str
     # Other accounts this same delivery also landed in (collapsed duplicates).
     also_in_account_ids: list[uuid.UUID] = []
+    # Bulk mail: the Subscriptions group it belongs to, how it can be left
+    # (one_click | mailto | web), and whether the user already did.
+    subscription_key: str | None = None
+    unsubscribe_via: str | None = None
+    unsubscribed: bool = False
+    # Its language, once found (the first time it is shown): "" unknown.
+    content_language: str = ""
 
 
 class ThreadDetail(BaseModel):
@@ -72,6 +81,7 @@ class ThreadDetail(BaseModel):
     order: str  # "asc" | "desc"
     pinned: bool
     messages: list[MessageEnvelope]  # always ASC (SPEC §9.4)
+    snoozed_until: dt.datetime | None = None
 
 
 class FlagChangeRequest(BaseModel):