feat: add iPad/iOS prototype via Tauri v2 mobile target

Initialize Tauri iOS support with conditional compilation:
- Desktop-only features (git CLI, menu, MCP, Claude CLI) gated behind #[cfg(desktop)]
- Mobile stubs return graceful errors so frontend degrades smoothly
- Vault read/write, AI chat, search, settings work unchanged on both platforms
- Xcode project generated, Rust cross-compiles cleanly to aarch64-apple-ios-sim
- All 581 Rust tests and 2201 frontend tests still pass

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/capabilities/default.json

@@ -6,6 +6,7 @@
     "main",
     "note-*"
   ],
+  "platforms": ["linux", "macOS", "windows"],
   "permissions": [
     "core:default",
     "core:window:allow-create",