Ship Android app by wrapping the web build with Capacitor (#20) (#28)

## Summary
- Closes #20
- Add Capacitor 7 to `llm-fe/` (`capacitor.config.ts`, appId `com.aimloperations.chat`, `webDir: build`) and commit the generated `android/` project
- npm scripts: `build:mobile` (`.env.mobile` + CRA build + `cap sync`), `android:open`, `android:sync`, `assets:generate`
- `.env.mobile` defaults to prod backend; override independently of web deploys
- Native chrome: Android back button (history / exit at root), status bar + keyboard resize, safe-area CSS; Preferences registered for JWT mirror (#22)
- Icons/splash via `@capacitor/assets`; signing via optional `keystore.properties`; docs in `llm-fe/ANDROID.md` + root README

Blockers #22 / #23 / #24 already on master.

## Test plan
- [x] `npm run test:ci` (79 tests)
- [x] `npm run build` + `npx cap add android` / `cap sync`
- [ ] `npm run build:mobile` + open in Android Studio on a machine with SDK
- [ ] Emulator/device QA: login, chat stream, theme, back button, keyboard/safe-area, WS resume
- [ ] Generate upload keystore outside repo; `./gradlew bundleRelease` for internal testing trackReviewed-on: #28

llm-fe/android/build.gradle

@@ -0,0 +1,29 @@
+// Top-level build file where you can add configuration options common to all sub-projects/modules.
+
+buildscript {
+    
+    repositories {
+        google()
+        mavenCentral()
+    }
+    dependencies {
+        classpath 'com.android.tools.build:gradle:8.7.2'
+        classpath 'com.google.gms:google-services:4.4.2'
+
+        // NOTE: Do not place your application dependencies here; they belong
+        // in the individual module build.gradle files
+    }
+}
+
+apply from: "variables.gradle"
+
+allprojects {
+    repositories {
+        google()
+        mavenCentral()
+    }
+}
+
+task clean(type: Delete) {
+    delete rootProject.buildDir
+}