chore(desktop): DESKTOP_APP_SUFFIX env for parallel-worktree dev

Dev Electron uses a single userData path ("Multica Canary") derived from
the app name, which also locates the single-instance lock. Two worktrees
running dev simultaneously fight for that lock — the second `app.quit()`s
silently before opening a window.

DESKTOP_APP_SUFFIX appends to the app name + userData path so each
worktree can claim its own lock:

  DESKTOP_APP_SUFFIX=foo  → "Multica Canary foo"

Default (no env var) keeps behavior unchanged.

Complements the existing DESKTOP_RENDERER_PORT env from #1210 so a full
"run a second dev Electron" setup looks like:

  DESKTOP_RENDERER_PORT=15173 DESKTOP_APP_SUFFIX=foo pnpm dev:desktop

apps/desktop/src/main/index.ts

@@ -109,7 +109,14 @@ function createWindow(): void {
 // is derived from the userData path. (Same approach VS Code uses for
 // Stable / Insiders coexistence.)
 
-const DEV_APP_NAME = "Multica Canary";
+// DESKTOP_APP_SUFFIX lets parallel worktrees run dev Electron side-by-side
+// without fighting for the shared single-instance lock. The suffix is
+// appended to the app name + userData path, so each worktree gets its own
+// lock file. Default (no env var) keeps behavior unchanged — the common
+// single-worktree case still lands at "Multica Canary".
+const DEV_APP_NAME = process.env.DESKTOP_APP_SUFFIX
+  ? `Multica Canary ${process.env.DESKTOP_APP_SUFFIX}`
+  : "Multica Canary";
 
 if (is.dev) {
   app.setName(DEV_APP_NAME);