fix(client/android): the build resolves cargo from CARGO_HOME before guessing ~/.cargo

The kts resolves cargo by ABSOLUTE path on purpose (a GUI Android Studio
launch has no ~/.cargo/bin on PATH), but user.home is the wrong anchor in
the CI image, where the shared toolchain lives at CARGO_HOME=/usr/local/cargo
— gradle died starting /root/.cargo/bin/cargo. CARGO_HOME/bin is where
rustup puts binaries whenever the variable is set, so it wins; the
~/.cargo fallback keeps GUI launches working.

Also: android.yml's path filter learns ci/android-ci.Dockerfile — an image
change must exercise its consumer instead of needing a manual rerun to
prove itself.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.gitea/workflows/android.yml

@@ -26,6 +26,9 @@ on:
     paths:
       - 'crates/**'
       - 'clients/android/**'
+      # The builder image is part of what this artifact is built from — an image
+      # change must exercise its consumer.
+      - 'ci/android-ci.Dockerfile'
       - 'Cargo.toml'
       - 'Cargo.lock'
       - 'rust-toolchain.toml'
@@ -39,6 +42,9 @@ on:
     paths:
       - 'crates/**'
       - 'clients/android/**'
+      # The builder image is part of what this artifact is built from — an image
+      # change must exercise its consumer.
+      - 'ci/android-ci.Dockerfile'
       - 'Cargo.toml'
       - 'Cargo.lock'
       - 'rust-toolchain.toml'

clients/android/kit/build.gradle.kts

@@ -47,7 +47,11 @@ dependencies {
 // /README.md): `cargo install cargo-ndk` + `rustup target add aarch64-linux-android x86_64-linux-android`.
 // ------------------------------------------------------------------------------------------------
 val repoRoot = rootDir.parentFile.parentFile // clients/android -> clients -> repo root
-val cargoBin = "${System.getProperty("user.home")}/.cargo/bin"
+// CARGO_HOME first: rustup puts every binary in $CARGO_HOME/bin, and the CI image
+// (ci/android-ci.Dockerfile) installs the shared toolchain at /usr/local/cargo — the
+// historical ~/.cargo fallback is what a GUI Android Studio launch (no env) still needs.
+val cargoBin = System.getenv("CARGO_HOME")?.let { "$it/bin" }
+    ?: "${System.getProperty("user.home")}/.cargo/bin"
 
 // SDK location without depending on AGP's DSL (sdkDirectory isn't in AGP 9's library extension):
 // env first (set by Android Studio and by our CLI shell), then local.properties, then the default.