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.

src-tauri/Cargo.lock

@@ -4439,7 +4439,6 @@ dependencies = [
  "hnsw_rs",
  "image",
  "kamadak-exif",
- "log",
  "memmap2",
  "mozjpeg",
  "notify",
@@ -4461,7 +4460,6 @@ dependencies = [
  "tokenizers",
  "tokio",
  "ureq",
- "uuid",
  "walkdir",
  "xxhash-rust",
  "zip 4.6.1",
@@ -6537,25 +6535,11 @@ dependencies = [
  "bytes",
  "libc",
  "mio 1.2.0",
- "parking_lot",
  "pin-project-lite",
- "signal-hook-registry",
  "socket2",
- "tokio-macros",
  "windows-sys 0.61.2",
 ]
 
-[[package]]
-name = "tokio-macros"
-version = "2.7.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "385a6cb71ab9ab790c5fe8d67f1645e6c450a7ce006a33de03daa956cf70a496"
-dependencies = [
- "proc-macro2",
- "quote",
- "syn 2.0.117",
-]
-
 [[package]]
 name = "tokio-native-tls"
 version = "0.3.1"