fix(android): give the app a home directory so it starts

backend/system resolves config and data from $HOME or the OS
equivalent, and Android has neither: buildUserDirPath switches on
runtime.GOOS with cases for darwin, linux and windows and a default
returning errUnsupportedOS. So NewYellowJacketApp failed and main()
called os.Exit(1) about six milliseconds after the JNI bridge came up.

That failure is invisible in all three places anyone would look. There
is no panic, no AndroidRuntime stack and no tombstone, because os.Exit
is not a crash; Go's stdout does not reach logcat, so the slog line
naming the error is discarded; and ActivityManager respawns the process
fast enough that pidof always answers, so a crash-looping app looks
alive.

main() now sets the override before anything asks for a path.
application.Mobile.StoragePath() is the platform's own answer --
getFilesDir() on Android, Application Support on iOS -- and returns ""
on desktop, where UseHomeOverride is a no-op, so this needs no build
tag and changes nothing off mobile. resolveUserDirPath already honours
YJ_HOME on every OS, so there was a seam for it.

The knowledge stays in main(): backend/system gains no import of the
Wails application package, for the same reason backend/events is split
by the indexbuild tag.

UseHomeOverride's two rules are tested because nothing else would
notice them breaking. An empty base does nothing, which is exactly the
desktop case. And an override already set wins, so YJ_HOME still
relocates a sandbox on the one platform that would otherwise decide for
itself.

This is not the end of the port. The app now reaches the database and
takes SIGSYS on the x86_64 emulator -- modernc.org/libc issues a raw
lstat syscall on linux/amd64 and Android's seccomp forbids it. arm64,
which is what ships to phones, has no lstat syscall at all and routes
through fstatat, so it is structurally unaffected. See NOTES.md.

backend/system/userdata.go

@@ -29,6 +29,29 @@ const (
 // without touching the current user's real config.toml or yj.db.
 const envHomeOverride = "YJ_HOME"
 
+// UseHomeOverride points every config and data path at base, by setting
+// the same override a development sandbox uses.
+//
+// It exists for mobile, where the switch in buildUserDirPath has no
+// answer: there is no home directory and no XDG, only a per-app private
+// directory the OS hands out at runtime. The caller is main(), which is
+// the only place that can ask the platform for it — deliberately, so
+// this package stays free of the Wails application package that knows
+// (see backend/events' indexbuild split for why that matters).
+//
+// Two rules. An empty base is a no-op, because that is exactly what
+// application.Mobile.StoragePath() returns on desktop. And an override
+// that is already set wins, so YJ_HOME on the command line still
+// relocates a sandbox on a platform that would otherwise decide for
+// itself.
+func UseHomeOverride(base string) {
+	if base == "" || os.Getenv(envHomeOverride) != "" {
+		return
+	}
+
+	_ = os.Setenv(envHomeOverride, base)
+}
+
 // getUserDirPath returns and creates the path for a user directory.
 func getUserDirPath(dt dirType) (string, error) {
 	path, err := resolveUserDirPath(dt)

backend/system/userdata_test.go

@@ -1,6 +1,7 @@
 package system
 
 import (
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -50,3 +51,39 @@ func TestResolveUserDirPath_NoOverrideUsesOSPath(t *testing.T) {
 		)
 	}
 }
+
+// UseHomeOverride carries two rules that a mobile launch depends on and
+// that nothing else would notice breaking: an empty base must do
+// nothing, because that is precisely what StoragePath() returns on
+// desktop, and an override already set must win, or YJ_HOME would stop
+// relocating a sandbox on the platform that decides for itself.
+func TestUseHomeOverride(t *testing.T) {
+	const (
+		storage = "/data/user/0/app.yellowjacket/files"
+		sandbox = "/tmp/sandbox"
+	)
+
+	tests := []struct {
+		name    string
+		already string
+		base    string
+		want    string
+	}{
+		{name: "empty base is a no-op", already: "", base: "", want: ""},
+		{name: "sets the override when unset", already: "", base: storage, want: storage},
+		{name: "an existing override wins", already: sandbox, base: storage, want: sandbox},
+		{name: "empty base keeps an existing override", already: sandbox, base: "", want: sandbox},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Setenv(envHomeOverride, tt.already)
+
+			UseHomeOverride(tt.base)
+
+			if got := os.Getenv(envHomeOverride); got != tt.want {
+				t.Errorf("%s = %q, want %q", envHomeOverride, got, tt.want)
+			}
+		})
+	}
+}