fix(gui): route runtime status messages through Loc instead of hardcoded Italian

Send/Donate/Sync/Wizard view models wrote status/error strings
directly in Italian ("Importo non valido.", "Errore: …", the TLS
pin-mismatch message, …), so a user running the app in English/German/
etc. still saw them in Italian despite the 6-language Loc system
covering the rest of the UI.

Added the missing Loc keys and routed every occurrence through
Loc.Tr(...). CertificatePinMismatchException (Core) no longer bakes an
Italian message into .Message; it now exposes Host/Port so the UI can
translate it, with an English fallback for any exception type the UI
doesn't specifically handle. Also drops the now-stale USERGUIDE.md
caveat about a few messages staying in Italian regardless of language.

USERGUIDE.md

@@ -583,8 +583,7 @@ The Settings overlay (gear icon / menu *Settings*) contains exactly two preferen
 the door to the server settings:
 
 - **Language** — Italiano, English, Español, Français, Português, Deutsch. Applies
-  immediately. Default: English. (A few low-level error messages are currently shown in
-  Italian regardless of language.)
+  immediately. Default: English.
 - **Amount unit** — how amounts are displayed *and interpreted when you type them*:
 
   | Unit | In satoshi | Decimals shown |