M6 hosted mailboxes (off by default); dark message mode; fixes

Hosted mailboxes (SPEC App. D), behind HOSTED_MAILBOXES (off: unchanged):
- Stalwart mail server, provisioned by `python -m app.hosted.provision`;
  directory read from a Postgres view, app passwords only for mail apps
- native account per user, the worker signs in as master for the user
- raw messages of hosted mailboxes only cached, fetched back on demand
- migrations 0025 (accounts.kind, directory view), 0026 (IMAP ids bigint)

Mail:
- dark message mode switch in the reading pane (Thunderbird's algorithm)
- moves and folder creation quote the folder name ("Deleted Items"):
  swipe-to-delete / Trash no longer bounce back
- thread unread counters: lock before recount, sorted lock order,
  UIDVALIDITY re-import recounts, nightly repair_threads
- sidebar accounts sorted alphabetically, more space above headings

Settings: Security headings renamed; antispam accounts are explicit
(none ticked = none filtered), input widths, Activity hidden.
Plugin bundles served with Cache-Control: no-cache.

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

backend/app/models.py

@@ -131,6 +131,10 @@ class Account(Base):
     imap_config: Mapped[dict[str, object]] = mapped_column(JSONB, nullable=False)
     smtp_config: Mapped[dict[str, object]] = mapped_column(JSONB, nullable=False)
     secret_ct: Mapped[bytes] = mapped_column(BYTEA, nullable=False)
+    # external: any IMAP/SMTP server (the user's password in secret_ct);
+    # native: the user's own mailbox on the hosted mail server — permanent, no
+    # stored password (SPEC App. D §D.4; app/hosted/).
+    kind: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("'external'"))
     discovery_src: Mapped[str] = mapped_column(Text, nullable=False)
     state: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("'backfilling'"))
     state_detail: Mapped[str] = mapped_column(Text, nullable=False, server_default=text("''"))
@@ -161,9 +165,12 @@ class Mailbox(Base):
     )
     path: Mapped[str] = mapped_column(Text, nullable=False)
     special_use: Mapped[str | None] = mapped_column(Text)
-    uidvalidity: Mapped[int] = mapped_column(Integer, nullable=False, server_default=text("0"))
-    highestmodseq: Mapped[int | None] = mapped_column(Integer)
-    last_seen_uid: Mapped[int] = mapped_column(Integer, nullable=False, server_default=text("0"))
+    # IMAP ids are unsigned 32-bit (MODSEQ 63-bit): bigint, not integer.
+    uidvalidity: Mapped[int] = mapped_column(BigInteger, nullable=False, server_default=text("0"))
+    highestmodseq: Mapped[int | None] = mapped_column(BigInteger)
+    last_seen_uid: Mapped[int] = mapped_column(
+        BigInteger, nullable=False, server_default=text("0")
+    )
 
 
 class Thread(Base):
@@ -214,7 +221,7 @@ class Message(Base):
     restore_mailbox_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("mailboxes.id", ondelete="SET NULL")
     )
-    uid: Mapped[int] = mapped_column(Integer, nullable=False)
+    uid: Mapped[int] = mapped_column(BigInteger, nullable=False)
     thread_id: Mapped[uuid.UUID | None] = mapped_column(
         UUID(as_uuid=True), ForeignKey("threads.id")
     )
@@ -321,7 +328,7 @@ class Draft(Base):
     payload: Mapped[dict[str, object]] = mapped_column(JSONB, nullable=False)
     # UID of the mirrored copy in the account's IMAP Drafts folder (so the draft
     # is visible/continuable on other mail clients). Replaced on each edit.
-    imap_uid: Mapped[int | None] = mapped_column(Integer)
+    imap_uid: Mapped[int | None] = mapped_column(BigInteger)
     updated_at: Mapped[dt.datetime] = _now()
 
 
@@ -367,6 +374,8 @@ class UserAppPassword(Base):
     created_at: Mapped[dt.datetime] = _now()
     # Set by the mail server's sign-in, once there is one.
     last_used_at: Mapped[dt.datetime | None] = mapped_column(TIMESTAMP(timezone=True))
+    # The hosted mail server's id of the same password (§D.5), when it has one.
+    server_id: Mapped[str | None] = mapped_column(Text)
 
 
 class PushSubscription(Base):