Record the live Files and signature verification

Both entries had been sitting as pending QA. Everything they were waiting on
has now run against the live 0.15.5 server: oversized, non-ASCII and
inline-image signatures (with a test message reaching Gmail intact), and folder
creation, listing, upload, rename, move and delete in Files.

The Files entry now describes what actually differs before 0.16 — the query
that cannot see folders, the missing nodeType, the coarser rights — since all
three were found the hard way and none of them surfaces as an error.

README.md

@@ -131,10 +131,10 @@ Press `?` anywhere. Highlights: `c` compose · `/` search · `j`/`k` navigate ·
 
 ## Known issues / pending QA
 
-Verified against the mock server and, for the core mail flows, against a live Stalwart 0.15.5. Still pending live verification:
+Verified against the mock server, and against a live Stalwart 0.15.5 for the mail flows, self-service credentials, Files and signatures.
 
-- **HTML signatures** — Stalwart caps identity signatures at 2 KB. ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker (other clients see a text fallback). The end-to-end flow (save → compose → send with inline logo) is implemented but not yet confirmed on the live server.
-- **Files** — the live server runs an older Stalwart build than `main`; `FileNode/query` there rejects `isTopLevel`/`parentId` filters, so ihasmail falls back to listing all nodes and building the tree client-side. Upload/rename/move/delete still need a live pass.
+- **HTML signatures** — Stalwart caps a signature at 2047 **bytes** (`value.len() < 2048` on a Rust string, so UTF-8 bytes, not characters). ihasmail compacts pasted HTML, moves images to Files and, if still too large, keeps the full signature in Files behind a short marker; other clients see a text fallback. Confirmed live on 0.15.5 (2026-08-24): oversized, non-ASCII and inline-image signatures all save, and a test message arrived intact at Gmail with the logo inline.
+- **Files on Stalwart before 0.16** — three things differ there, none of which the server reports as an error. `FileNode/query` masks its results to non-containers, so it returns files and **never folders**; `nodeType` does not exist, and sending it fails the create outright (a directory is instead a node with no file properties at all); and rights are only `mayRead`/`mayWrite`/`mayShare`, so the finer-grained `mayDelete`/`mayRename` the UI gates on are absent. ihasmail detects the older server by the absence of `urn:stalwart:jmap`, lists the tree through `FileNode/get` instead of query, shapes creates accordingly, and widens the old rights. Upload, folder creation, listing, rename, move and delete are all confirmed live on 0.15.5 (2026-08-24).
 - **Self-service credentials** — the **0.15.x REST path is confirmed live** against Stalwart 0.15.5 (2026-08-24): password change, app passwords, and enabling and disabling 2FA, on a real mailbox. The **0.16 registry path has only been exercised against the mock**, which enforces the same rules a real server does (current password required, password policy, a TOTP code on every request once 2FA is on, app passwords exempt from it) — it still wants a pass against a real 0.16 server. Password changes are refused by Stalwart for accounts backed by an external directory (LDAP/SQL/OIDC); the server's own message is shown when that happens.
 - Recurring events: colour/category/edit/delete apply to the whole series (per-occurrence overrides aren't supported by the server yet).
 - Editable date boxes are always Gregorian and in Latin digits, even for locales whose *display* uses another calendar or numbering system (`fa-IR`, `th-TH`, `ar-EG`) — they keep the locale's field order and separator, but a Buddhist-era year in a text box does not round-trip against the Gregorian calendar grid. Non-Gregorian calendar support is not implemented.