Retry other resolved addresses when a source host is partially blocked

[Zmeikich]

Problem

Some ISPs (notably in Russia) block individual IP addresses of *.githubusercontent.com.
gist.githubusercontent.com resolves to four addresses (185.199.108.133 – 185.199.111.133),
and when only some of them are blocked, the Github source fails intermittently:

Failed to fetch available localizations: Request error
Caused by:
0: error sending request for url (https://gist.githubusercontent.com/kimght/322a2779922ab5a5f96ff7f7dc3f8e82/raw/localizations.json)
1: client error (Connect)
2: os error 10054 (connection reset by peer)

Whether a request succeeds depends purely on which address the OS resolver happens to
return, so the app appears "randomly broken" for affected users. The same issue affects
localization downloads and fonts (release-assets.githubusercontent.com,
raw.githubusercontent.com share the same address pool).

Fix

• When a request fails to connect (or times out), the failing host is re-resolved and the
  request is retried with each address pinned in turn (5 s connect timeout per address)
  until one works. The failing host is taken from the reqwest error, so redirect targets
  (e.g. github.com → release-assets.githubusercontent.com) are handled too.
• The shared client got a 10 s connect timeout so a blocked address can't eat the whole
  30 s request timeout.
• While addresses are being cycled, the backend emits a connection_status event and the
  localizations loading screen shows what is happening instead of a bare spinner
  ("Can't connect directly, trying 185.199.111.133:443"). Messages are translated for all
  supported languages.
• The first working address found for a host is saved to address_cache.json in the
  app config directory and used for subsequent requests and launches, so users on
  affected networks only wait through the discovery once. If a saved address stops
  working, it is dropped and rediscovered automatically.

Testing

Tested on a real network where the ISP blocks 185.199.110.133:

WARN Request to https://gist.githubusercontent.com/... failed to connect, retrying each address of gist.githubusercontent.com
WARN Address 185.199.110.133:443 of gist.githubusercontent.com failed
INFO Connected to gist.githubusercontent.com via 185.199.111.133:443

Previously the Github source failed whenever the resolver picked the blocked address;
now the list, downloads and fonts recover ~5 s after the first failed attempt, with the
status line visible during the retry. Also verified that behavior on unaffected networks is unchanged
(cargo check and pnpm build pass, no new warnings).
Restarting the app after discovery connects through the saved address immediately
(Loaded saved addresses in the log, no retry delay).