feat(skills): read-only built-in skills; fix(chat): allow CSV attachments

[squarezw]

Read-only built-in skills, and CSV attachments in chat

Two independent changes, both small and both front-end only.

1. Read-only state for built-in skills

The backend gains is_managed — skills that ship with the platform and sync
from the repo on each release. Users cannot edit or delete them.

The UI says why rather than making buttons vanish. A missing button reads as
a permission problem, and a user will go ask an admin for access that nobody
has. So the list shows a 「平台维护」 badge and a 查看 button, and the editor
drops its write actions instead of disabling them — a disabled button still
invites clicking.

canEditSkill checks is_managed first, ahead of the super-admin branch.
Super admins cannot edit these either: the constraint is not about privilege
level, it is that any edit gets silently overwritten by the next sync.

Falls back safely if the backend is older — is_managed undefined means the
skill is treated as ordinary.

2. CSV attachments in chat

CSV was rejected in chat although the backend has supported it all along
(file_processing_service.supported_extensions has a .csv handler, and
upload-confirm already labels the type). Nothing on the server side blocked
it; the presign endpoint does not check types at all.

Two front-end gates did, and they were two separately-maintained lists:
the accept attribute on the file input, and a MIME allowlist in
useFileAttachments. Both had drifted and both were missing .csv. Adding
the extension to only one of them produces a file you can pick but cannot
upload — so they are now merged into lib/chatAttachments.ts, with accept
derived from the extension list. They cannot diverge again, and a test asserts
it.

Validation is now extension-first with MIME as fallback. file.type comes
from the OS type registry and differs per machine for CSV: text/csv on
macOS, application/vnd.ms-excel on Windows with Excel installed, sometimes
an empty string. A MIME-only check would work on one machine and fail on a
colleague's with nothing in the error to explain why. The .ai special case
existed for exactly this reason; it is now the general rule rather than a
per-type patch.

MIME is kept as a fallback for files with no extension — pasted screenshots.

The list is aligned with the backend's supported extensions: anything that
uploads can be read by extract_document_text.

Tests

244 passed (was 229). tsc reports 135 errors against a baseline of 134; the
extra one is TS5097 on the new test file, the same error the repo's other 18
test files already produce (tests run under --experimental-strip-types, which
requires the .ts suffix that tsc rejects).

Mutation-verified: dropping .csv from the list turns 4 red; reverting to
MIME-only turns 6 red; changing endsWith to includes (which would let
note.csv.exe through as a CSV) turns 1 red.

Note on pairing

The read-only half pairs with the backend PR that adds is_managed. Merging
this first is harmless — the badge simply does not appear until the backend
ships. The CSV half is independent of both.

🤖 Generated with Claude Code