fix(core,gui,cli): a blocked sign-in no longer asks to update the app (B209)

Proton answers a sign-in it blocks for abuse or rate limits with code
2028 and the text "This version of the app is no longer supported". The
SDK read 2028 as Unknown, so the app passed that text through and told
users to update a current release (issues #28, #39).

- deps: proton-sdk and proton-drive-rs 0.8.3, which name 2028 Blocked
- core: error::sign_in_blocked detects it
- gui: the login page says Proton is refusing sign-in for now and that
  waiting helps, translated in every catalog
- cli: pdfs login adds the same hint to the error
- cli: pdfs diagnose warns instead of failing when cache.db does not
  exist yet; the daemon creates it after the first sign-in
- docs: BUGS.md B209, changelog, troubleshooting entry

docs/TROUBLESHOOTING.md

@@ -88,6 +88,13 @@ Quit the app first, so this starts a new instance. The page then runs without We
 so use this only for the sign-in. The stored session keeps working without it. Signing in from
 your usual network, without a VPN, often avoids the CAPTCHA in the first place.
 
+**"This version of the app is no longer supported"** or **"Proton is temporarily refusing
+sign-in from this device"** — Proton blocks sign-in for a while after too many attempts or from a
+network it distrusts, and sends the first message even when the release is current. Do not retry
+for several hours, up to a day, then sign in once. If it still fails, run
+`RUST_LOG=debug pdfs login` and look for `code=` in the `proton api request failed` line: 2028 is
+this block, 5003 is a release Proton no longer accepts.
+
 **Sign-in works but is forgotten after a restart** — no Secret Service provider is running, or its
 keyring is locked. Install and unlock GNOME Keyring, KWallet or KeePassXC (with Secret Service
 enabled). `pdfs diagnose` shows `keyring session` as failing in that case.