build(deps): gate CUDA behind default feature, add CPU build scripts

Applies the stashed 'deps and cpu mode' work: cuda was hardcoded on the
candle dependency lines, making the candle-cuda feature flag dead and any
non-CUDA build impossible. Now default = [candle-cuda] for the main dev
machine, with pnpm dev:app:cpu / build:app:cpu (--no-default-features) for
CPU-only machines and CI. Also trims unused deps (log, uuid, tokio-full,
chrono-serde). Verified: cargo check --locked --no-default-features passes.

package.json

@@ -9,6 +9,8 @@
     "build:vite": "tsc && vite build",
     "clean:app": "cd src-tauri && cargo clean",
     "dev:app": "tauri dev",
+    "dev:app:cpu": "tauri dev -- --no-default-features",
+    "build:app:cpu": "tauri build -- --no-default-features",
     "dev:vite": "vite",
     "preview": "vite preview",
     "tauri": "tauri"