Two-way draft sync, signatures, sent/bcc fixes, and mail-list UI

Backend
- Sent copies: persist locally on send + periodic multi-folder CONDSTORE poll
  (Sent/Archive/Trash/Junk) so they thread and show without an IDLE-only INBOX.
- Bcc: transmitted copy omits Bcc, Sent copy keeps it; store/display bcc_addr.
- Drafts: mirror local drafts to the IMAP Drafts folder (append/replace/expunge);
  inbound sync of foreign drafts (un-threaded) with adopt-to-edit + dedup by UID.
- Per-user storage quota (5 GiB default) and manual "sync all folders" action.
- Account editing (display/color/server/password) and per-account HTML signature.
- Account view scoped to that account's inbox; new migrations 0006–0009.

Frontend
- Mail-list pagination, folder filters (All/Unread/Pinned), sort toggle, refresh.
- Aligned + widened mail-list margins; reading-pane bars fixed height.
- Rich-text composer: color palette, highlight, image; attachments; expand/pop-out.
- Signatures: auto-insert for new mail, replace on From change, reply insert.
- Restore/Delete actions per folder; drafts view merges local + remote, date-sorted.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api/settings.py

@@ -6,11 +6,12 @@ from typing import Annotated
 
 from fastapi import APIRouter, Depends
 from pydantic import BaseModel
+from sqlalchemy import func, select
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.db import get_session
 from app.identity.deps import CurrentUser, require_csrf
-from app.models import User
+from app.models import Message, User
 
 router = APIRouter(prefix="/settings", tags=["settings"])
 
@@ -30,6 +31,11 @@ class SettingsPatch(BaseModel):
     tz: str | None = None
 
 
+class QuotaOut(BaseModel):
+    used_bytes: int
+    quota_bytes: int
+
+
 def allowed_image_senders(user: User) -> set[str]:
     value = user.settings.get("allowed_image_senders", [])
     if isinstance(value, list):
@@ -59,6 +65,15 @@ async def get_settings_endpoint(user: CurrentUser) -> SettingsOut:
     return _settings_out(user)
 
 
+@router.get("/quota", response_model=QuotaOut)
+async def get_quota(user: CurrentUser, db: DbSession) -> QuotaOut:
+    """Storage quota and current usage (sum of stored message sizes)."""
+    used = await db.scalar(
+        select(func.coalesce(func.sum(Message.size_bytes), 0)).where(Message.user_id == user.id)
+    )
+    return QuotaOut(used_bytes=int(used or 0), quota_bytes=user.quota_bytes)
+
+
 @router.patch("", response_model=SettingsOut, dependencies=[Depends(require_csrf)])
 async def patch_settings(body: SettingsPatch, user: CurrentUser, db: DbSession) -> SettingsOut:
     updates = dict(user.settings)