Pin notes to a chat, and import notes from files

Two additions to the shared notes, both about getting content in and out of
the prompt.

Per-chat pinning. Notes normally reach the model as a title list only, and
the body arrives just if the model decides to call read_note. A 📌 on the tab
now puts a page's full body in the system prompt for the chat that is open,
every turn, no tool round trip. The selection is per session (new table
session_note_pages, unique on session_id + page_id), so different
conversations carry different pages. The title list marks pinned pages so the
model does not read them twice, and bodies are capped per page by
SESSION_NOTES_CONTENT_LIMIT.

A new chat has no session row until its first message, so the front end
cannot PUT the selection yet. POST /chat/stream therefore accepts
note_page_ids and applies it before building the context, which makes the
pins count from the very first turn.

File import. Dropping .md/.txt on the editor - or picking them with the new
⬆ button - creates pages, and a .zip is unpacked into one page per text
entry with the first path component as the section. That is exactly the
shape the export writes, so a downloaded zip round-trips back in. Titles that
collide get a " (2)" suffix rather than overwriting, since restoring a backup
into a live workspace should not destroy anything. UploadFile needs
python-multipart, added to the lockfile.

Toasts grew a kind: the import success message was rendering in the red error
toast, as was the existing session-import message. Both are green now.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/uv.lock

@@ -1300,6 +1300,7 @@ dependencies = [
     { name = "nvidia-ml-py" },
     { name = "psutil" },
     { name = "python-dotenv" },
+    { name = "python-multipart" },
     { name = "sqlalchemy" },
     { name = "sse-starlette" },
     { name = "trafilatura" },
@@ -1336,6 +1337,7 @@ requires-dist = [
     { name = "pytest", marker = "extra == 'dev'", specifier = ">=8.0.0" },
     { name = "pytest-asyncio", marker = "extra == 'dev'", specifier = ">=0.23.0" },
     { name = "python-dotenv", specifier = ">=1.0.0" },
+    { name = "python-multipart", specifier = ">=0.0.9" },
     { name = "sqlalchemy", specifier = ">=2.0.0" },
     { name = "sse-starlette", specifier = ">=2.1.0" },
     { name = "trafilatura", specifier = ">=2.1.0" },
@@ -1642,6 +1644,15 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/0b/d7/1959b9648791274998a9c3526f6d0ec8fd2233e4d4acce81bbae76b44b2a/python_dotenv-1.2.2-py3-none-any.whl", hash = "sha256:1d8214789a24de455a8b8bd8ae6fe3c6b69a5e3d64aa8a8e5d68e694bbcb285a", size = 22101, upload-time = "2026-03-01T16:00:25.09Z" },
 ]
 
+[[package]]
+name = "python-multipart"
+version = "0.0.32"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/5b/42/55c32bb9b12693c092ad250a0e82edb5b31ddeda6eb772de5f308b3804ad/python_multipart-0.0.32.tar.gz", hash = "sha256:be54b7f3fa167bb83e4fcd936b887b708f4e57fe75911c02aebf53efaf8d938e", size = 46881, upload-time = "2026-06-04T16:18:58.647Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/e1/04/e8135ebd1ad02c56ec633277529b2602ff99ff634be76cdba5744cf554fd/python_multipart-0.0.32-py3-none-any.whl", hash = "sha256:ff6d3f776f16878c894e52e107296ffc890e913c611b1a4ec6c44e2821fe2e23", size = 30042, upload-time = "2026-06-04T16:18:57.319Z" },
+]
+
 [[package]]
 name = "pytz"
 version = "2026.2"