feat(06-01): scaffold apps/desktop as @tessera/desktop Tauri 2.x project

- Create Tauri project structure with Cargo.toml, tauri.conf.json, build.rs
- Implement lib.rs with store plugin and server URL navigation on startup
- Configure capabilities with core:default and store:default permissions
- Set frontendDist to ../src for local setup page (no bundled frontend)
- Add placeholder icons for build compatibility (to be replaced in 06-02)
- Add Cargo target/ to .gitignore
- Window config: 1280x800, centered, native decorations, resizable

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/desktop/src-tauri/capabilities/default.json

@@ -0,0 +1,10 @@
+{
+  "$schema": "../gen/schemas/desktop-schema.json",
+  "identifier": "default",
+  "description": "Tessera desktop capabilities",
+  "windows": ["main"],
+  "permissions": [
+    "core:default",
+    "store:default"
+  ]
+}