v1.2.9: fix UI build for youtube_via_relay (v1.2.8 CI abandoned)

v1.2.8 tagged cleanly but CI failed compiling mhrv-rs-ui with:

  error[E0063]: missing field `youtube_via_relay` in initializer of
  `mhrv_rs::config::Config`

When I added the youtube_via_relay field to the main Config struct
in 09f1f5f, I missed the struct-literal construction in src/bin/ui.rs
(FormState::save_to_config) and the ConfigWire serializer.

Fixed here:

- Added youtube_via_relay field to FormState (line 214), read path
  (line 291), default path (line 316), and the save path (line 451)
- Added youtube_via_relay field to ConfigWire (line 493) with
  skip_serializing_if on false, plus its From impl (line 544)

UI still doesn't expose a checkbox for the toggle — it's config-only
for now, same treatment as normalize_x_graphql. A future PR can add
the checkbox to the Advanced pane.

v1.2.8 tag exists but has no GitHub Release (release job skipped
on failure); v1.2.9 is the clean cut. Same payload as v1.2.8 plus
this fix.

android/app/build.gradle.kts

@@ -14,8 +14,8 @@ android {
         applicationId = "com.therealaleph.mhrv"
         minSdk = 24 // Android 7.0 — covers 99%+ of live devices.
         targetSdk = 34
-        versionCode = 128
-        versionName = "1.2.8"
+        versionCode = 129
+        versionName = "1.2.9"
 
         // Ship all four mainstream Android ABIs:
         //   - arm64-v8a      — 95%+ of real-world Android phones since 2019