v0.8.4: config load failures are no longer silent (diagnoses the reset-on-reopen bug)

A user reported that after Save-config, closing the UI, and reopening,
every form field was blank — but the config.json on disk still had all
the right values.

The culprit in the UI was load_form()'s swallow-errors pattern:

  let existing = if path.exists() {
      Config::load(&path).ok()   // .ok() threw away the error
  } else { ... };
  if let Some(c) = existing { /* populate form */ } else { /* defaults */ }

When Config::load returned an Err, .ok() silently converted to None,
the form went back to defaults, and the user had no signal at all
that the load had failed or WHY. On every platform I could test
(macOS / Linux) the round-trip works fine with a real round-trip test
added in config.rs (config::rt_tests::round_trip_all_current_fields
and round_trip_minimal_fields_only — both green). So whatever's
failing for this specific reporter is environment-specific (weird
filesystem encoding, partial write, different field shape from an
older version, … TBD). Without visibility we can't diagnose it.

Changes:

1. load_form() now returns (FormState, Option<String>). The String
   is a user-facing error message (with the full path + the
   underlying parse/validate reason) when Config::load fails on an
   existing file.
2. main() plumbs that error into App's initial toast, which sticks
   for 30 seconds (vs the normal 5 for regular toasts) so users who
   only open the UI briefly still see it.
3. Added tracing::info! in load_form for the success path too —
   the Recent log panel now always shows either 'config: loaded OK
   from <path>' or 'Config at <path> failed to load: <reason>' on
   startup, regardless of toast timing.
4. Added two regression-guard tests in config.rs covering the
   full-fields and minimal-fields save shapes the UI emits.

Next time a user reports this: they'll have the exact error in the
toast + the Recent log panel, and we can fix the actual bug instead
of shooting blind.

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mhrv-rs"
-version = "0.8.3"
+version = "0.8.4"
 edition = "2021"
 description = "Rust port of MasterHttpRelayVPN -- DPI bypass via Google Apps Script relay with domain fronting"
 license = "MIT"