M3 plugin platform: frontend host, reference plugins, UI kit + docs

Completes M3 (SPEC §11 + §12 acceptance) on top of the backend platform (4e077f1).

Frontend host (§11.3):
- src/plugins/: manifest loader, runtime ESM bundle loader (framework-agnostic
  mount contract), PluginSlot, PluginPage (app.page at /p/*), plugin context
  (scoped api/events/i18n), capabilities hook. Wired slots into ThreadView
  (message.actions/message.banner), MailLayout sidebar (sidebar.section + page nav)
  and Settings→Accounts (settings.page); ws.ts fans out core + plugin.* events.
- plugin-ui.css: theme-aware .wm-* classes so bundles don't re-derive inline styles.

SDK additions:
- CurrentUserId + require_csrf (plugin-router auth), messages.envelope/header,
  actions.set_flags/trash, sdk.enqueue.

Reference plugins (plugins/, installed via webmail.plugins entry points):
- hello: skeleton page-plugin (app.page + plg_hello_note w/ RLS + cron job) —
  install→run→uninstall --drop-data leaves zero residue.
- labels: message.actions + sidebar.section slots, KV only, zero core diffs.
- filters: message.new hook -> match (subject/from/to) -> Mark read / Pin / Delete
  (delete = move thread to Trash via the core path).

docs/plugins.md: full authoring guide + a complete "Unsubscribe button" worked
example (the §12 "writable from docs alone" bar).

Also: conftest resets the arq pool between tests (loop-bound global, like engine/redis).

Gates: backend ruff + mypy --strict (107) + full suite 337; frontend tsc + eslint
+ vitest (11) + vite build.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Rcj3TpydMTzTSmU7kunzf8

backend/tests/plugins/test_labels_plugin.py

@@ -0,0 +1,102 @@
+"""End-to-end test of the labels reference plugin (SPEC §11, §12 M3 acceptance).
+
+Slots + KV only, zero core diffs. Verifies the real HTTP surface (discovery ->
+router -> auth -> KV) and that labels are per-user isolated by construction: one
+user never sees another's labels or label set. Requires the plugin installed.
+"""
+
+from __future__ import annotations
+
+import uuid
+from collections.abc import AsyncIterator
+
+import httpx
+import pytest_asyncio
+from httpx import ASGITransport
+from sqlalchemy import delete
+from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
+from sqlalchemy.pool import NullPool
+
+from app.config import get_settings
+from app.identity.passwords import hash_password
+from app.identity.sessions import create_session
+from app.main import create_app
+from app.models import User
+
+
+class _Ctx:
+    users: list[uuid.UUID]
+    client_a: httpx.AsyncClient
+    client_b: httpx.AsyncClient
+
+
+@pytest_asyncio.fixture
+async def ctx() -> AsyncIterator[_Ctx]:
+    engine = create_async_engine(get_settings().database_url, poolclass=NullPool)
+    sm = async_sessionmaker(engine, expire_on_commit=False)
+    c = _Ctx()
+    c.users = []
+    async with sm() as s:
+        for _ in range(2):
+            user = User(
+                handle=f"lb{uuid.uuid4().hex[:8]}@test.local", password_hash=hash_password("pw")
+            )
+            s.add(user)
+            await s.flush()
+            c.users.append(user.id)
+        await s.commit()
+
+    transport = ASGITransport(app=create_app())
+
+    async def client(uid: uuid.UUID) -> httpx.AsyncClient:
+        token = await create_session(uid, "pytest")
+        return httpx.AsyncClient(
+            transport=transport,
+            base_url="http://testserver",
+            cookies={"session": token, "csrf": "csrf"},
+            headers={"X-CSRF": "csrf"},
+        )
+
+    c.client_a = await client(c.users[0])
+    c.client_b = await client(c.users[1])
+    try:
+        yield c
+    finally:
+        await c.client_a.aclose()
+        await c.client_b.aclose()
+        async with sm() as s:  # cascade drops the users' plugin_kv rows
+            await s.execute(delete(User).where(User.id.in_(c.users)))
+            await s.commit()
+        await engine.dispose()
+
+
+async def test_labels_crud_and_per_user_isolation(ctx: _Ctx) -> None:
+    mid = "msg-1"
+    msg_path = f"/api/plugins/labels/message/{mid}/labels"
+
+    # A labels a message; reads it back; the label joins A's known set.
+    await ctx.client_a.put(msg_path, json={"labels": ["work", "urgent"]})
+    assert (await ctx.client_a.get(msg_path)).json() == [
+        "urgent",
+        "work",
+    ]
+    assert (await ctx.client_a.get("/api/plugins/labels/labels")).json() == ["urgent", "work"]
+
+    # B sees neither A's per-message labels nor A's label set (KV is per-user).
+    assert (await ctx.client_b.get(msg_path)).json() == []
+    assert (await ctx.client_b.get("/api/plugins/labels/labels")).json() == []
+
+    # Empties/whitespace are dropped and the set is deduped/sorted.
+    await ctx.client_a.put(msg_path, json={"labels": ["work", " ", "later"]})
+    assert (await ctx.client_a.get(msg_path)).json() == ["later", "work"]
+    assert (await ctx.client_a.get("/api/plugins/labels/labels")).json() == [
+        "later",
+        "urgent",
+        "work",
+    ]
+
+
+async def test_labels_requires_auth(ctx: _Ctx) -> None:
+    transport = ASGITransport(app=create_app())
+    async with httpx.AsyncClient(transport=transport, base_url="http://testserver") as anon:
+        assert (await anon.get("/api/plugins/labels/labels")).status_code == 401