feat(android): bundle.android config + frontend isAndroid/isMobile helpers

Two small Phase 1 follow-ups for the Android target:

1. tauri.conf.json: add `bundle.android.minSdkVersion: 24`. Android 7.0
   is the floor — gives us Vulkan availability (for the eventual GPU
   feature flag), AAudio for cpal, and is what Pixel-class hardware
   tests against. Keeps the global `identifier` on `uk.co.corbel.kon`
   for now; the Corbie rebrand sweep will land `corbel.technology.corbie`
   as a single coherent commit.

2. src/lib/utils/runtime.ts: add `isAndroid()` and `isMobile()` helpers
   alongside the existing `hasTauriRuntime()`. Both use UA sniffing —
   sufficient for feature-gating UI, never for security decisions.
   These are how the Svelte side will hide:
   - hotkey config (no global hotkey API on Android)
   - paste-mode picker (auto-paste maps to a copy-only flow)
   - meeting auto-capture toggle (process list unavailable)
   - multi-window buttons (open-viewer, open-float, etc.)
   - system-tray-related affordances

Tauri 2 doesn't expose a synchronous platform-detection helper that
works during initial render, so UA sniffing is the pragmatic choice.

https://claude.ai/code/session_0189xUb6ie6t9qHkzatGZ9Rb

src-tauri/tauri.conf.json

@@ -35,6 +35,9 @@
       "icons/128x128@2x.png",
       "icons/icon.icns",
       "icons/icon.ico"
-    ]
+    ],
+    "android": {
+      "minSdkVersion": 24
+    }
   }
 }