M10.2: Tauri v2 desktop scaffold (desktop/) — Vue frontend as the webview

New desktop/ Tauri v2 project (Linux-first, cross-platform-ready):
- src-tauri: Cargo.toml (lib + thin main.rs shim), build.rs, lib.rs (Builder
  entry point), tauri.conf.json (frontendDist -> ../../frontend/dist, devUrl
  :5173, deb+appimage bundles), capabilities/default.json (core:default),
  .gitignore.
- The shared Vue 3 frontend is the sibling ../frontend; before-commands cd via
  "$(git rev-parse --show-toplevel)/frontend" since frontend and src-tauri are
  siblings, not nested.
- Icons generated from frontend/public/icon.svg via `cargo tauri icon` in CI
  (M10.8), not committed.

Boots the shared UI in a native window. The local data adapter (M10.3/M10.5)
and CI build verification (M10.8) follow. desktop/** is not yet in the CI paths
filter — added with the desktop lane in M10.8.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FRgehjoz7Yv8LkUfADxACm

desktop/src-tauri/build.rs

@@ -0,0 +1,3 @@
+fn main() {
+    tauri_build::build()
+}