Add Android support via Tauri

The Rust core was already portable (caller-provided db paths, rustls rather
than openssl) and the shell already had the mobile entry point, so this is
mostly toolchain, provisioning, and platform-gating.

Toolchain:
- scripts/android-setup.sh installs what Android Studio doesn't: SDK
  cmdline-tools, a pinned stable NDK, and the Rust Android targets. The NDK is
  pinned rather than "latest" because sdkmanager's newest is often a beta
  (30.0.15729638 is r30-beta2) -- not something to compile SQLite with. Prefers
  Android Studio's bundled JDK 21, since Gradle rejects newer JDKs.
- Track src-tauri/gen/android (excluding build output): AndroidManifest.xml is
  hand-edited and Android permissions have no tauri.conf.json equivalent.
- vite: serve on TAURI_DEV_HOST so a device/emulator can reach the dev server.

Card data: downloading streams ~740MB to build a ~73MB db, which is rough on a
phone. Add import_reference_db to adopt a desktop-built file instead, validated
by db::inspect_reference_db first so picking the wrong file is a clean error
rather than a broken install. Installs via copy-then-rename so an interrupted
copy can't leave a partial db.

Ingest hardening (helps desktop too):
- Give ureq connect/read timeouts. Cancellation is only polled per parsed
  record, so a stalled socket previously hung forever with no way to cancel.
- Drop the remove_file before rename; POSIX rename already replaces atomically,
  and the delete-first window could leave no reference.db at all if the process
  was killed -- much likelier on Android.

UI: hide Open/New Library on mobile (scoped storage has no arbitrary-path
equivalent, and SQLite can't open a content:// URI), reported via a cfg!(mobile)
flag on Status rather than user-agent sniffing. Use 100dvh so the soft keyboard
doesn't break the shell.

Verified: 70/70 core tests, desktop build clean, and the aarch64 APK builds
(bundled SQLite C and ring both cross-compile under NDK 28). Not yet run on a
device.

crates/mtgc-core/src/ingest.rs

@@ -152,7 +152,11 @@ pub fn build_reference_db(
     let result = build_into(&tmp, cancel, &mut progress);
     match &result {
         Ok(_) => {
-            let _ = std::fs::remove_file(dest);
+            // Rename straight over `dest`: POSIX rename replaces the
+            // destination atomically, so there is never a moment with no
+            // reference.db. (Deleting `dest` first would open a window where a
+            // process kill — far likelier on Android, which reaps backgrounded
+            // apps — leaves the user with no card data at all.)
             std::fs::rename(&tmp, dest)?;
         }
         Err(_) => {
@@ -331,8 +335,23 @@ struct BulkUris {
     default_size: u64,
 }
 
+/// HTTP agent with timeouts. Without them a stalled connection hangs forever:
+/// cancellation is only polled per *parsed record*, so if no bytes arrive, no
+/// records parse and `CancelToken` is never consulted — the ingest wedges with
+/// no way out. Mobile networks stall like this routinely.
+///
+/// `timeout_read` is per read call, not for the whole transfer, so a
+/// multi-hundred-megabyte download is fine as long as it keeps making progress.
+fn agent() -> ureq::Agent {
+    ureq::AgentBuilder::new()
+        .timeout_connect(std::time::Duration::from_secs(30))
+        .timeout_read(std::time::Duration::from_secs(60))
+        .build()
+}
+
 fn fetch_bulk_uris() -> Result<BulkUris> {
-    let index: BulkIndex = ureq::get(BULK_INDEX_URL)
+    let index: BulkIndex = agent()
+        .get(BULK_INDEX_URL)
         .set("User-Agent", USER_AGENT)
         .set("Accept", "application/json")
         .call()?
@@ -356,7 +375,7 @@ fn fetch_bulk_uris() -> Result<BulkUris> {
 }
 
 fn http_reader(url: &str) -> Result<impl Read> {
-    let resp = ureq::get(url).set("User-Agent", USER_AGENT).call()?;
+    let resp = agent().get(url).set("User-Agent", USER_AGENT).call()?;
     Ok(BufReader::with_capacity(1 << 20, resp.into_reader()))
 }