Make the Android app build, and fix what compiling it exposed

Why: the module was written but never compiled, so nothing about it was
proved. Building it found a name clash and a theme that would have crashed
every device below Android 10.

What:
- KeepOrgProvider.requireContext -> ctx(): ContentProvider gained
  requireContext() in API 30, and Kotlin refused the silent hiding.
- The application theme was @android:style/Theme.DeviceDefault.DayNight,
  which is API 29 while minSdk is 26. Replaced with @style/AppTheme, plain
  DeviceDefault in values/ and DayNight in values-v29/.
- Backup exclusions: data_extraction_rules.xml and backup_rules.xml exclude
  every domain for both cloud backup and device transfer. The session token
  authenticates as the user and the mirrored files are the organization's
  password databases; neither belongs in a backup.
- User-facing strings moved from code into strings.xml, so the app is
  translatable. Layouts stay in code, as before.
- mipmap-anydpi-v26 -> mipmap-anydpi (the qualifier is redundant at minSdk
  26), and the redundant activity label is gone.
- Lint is configured to write a text report, and `make android-lint` runs it;
  `make android` now fails with a clear message when ANDROID_HOME has no SDK.

Proved: the unsigned release APK builds (656 KB), lint reports "No issues
found", and the packaged DEX contains all 16 classes including every manifest
component. Toolchain used: JDK 21, Gradle 8.10.2, SDK platform 35 and
build-tools 35.0.0; on this arm64 host Google's x86-64 aapt2 runs under
qemu-user-static with amd64 multiarch libraries, which android/README.md now
documents. The app has still not run on a device: there is none here.

docs/HANDOVER.md

@@ -272,6 +272,7 @@ manual, never automatic:
 
 ```sh
 make android          # unsigned APK, named cc.tiredsysadmin.android.keeporg_<code>.apk
+make android-lint     # kept at zero findings
 make android-publish  # scp to the private F-Droid unsigned and metadata directories
 ```
 
@@ -298,16 +299,22 @@ What the suite actually covers:
   server-wins, client-wins, manual, read-only restore, server-side rename,
   and an update arriving over the event stream.
 
+The Android module is checked by `make android-lint`, which is kept at zero
+findings, and `make android` builds the unsigned release APK. See
+[../android/README.md](../android/README.md) for its requirements, including
+what an arm64 Linux build host needs.
+
 ## Known limits and the next things to do
 
 - **One server node.** The JSON store is single-writer by design. Horizontal
   scale means replacing `internal/store`.
 - **No entry-level merge.** See the strategies section. The pluggable place
   is the client, which holds the key.
-- **The Android module has not been compiled in this environment** (no JDK or
-  Android SDK was available). The Gradle wrapper is deliberately not
-  committed; run `gradle wrapper --gradle-version 8.9` once, then
-  `make android`. Expect to fix small things on the first build.
+- **The Android app has never run on a device.** It compiles clean, lint
+  reports no issues and the DEX carries every class the manifest names, but
+  no emulator or phone was available here. The first real run is where to
+  confirm the file-picker integration, the save-triggered sync and the
+  foreground notification.
 - **No key-file or Yubikey handling**, and none needed: those live in the
   KeePass application.
 - **Sessions are not listable or revocable individually** beyond