fix(proofing): use macOS NSSpellChecker for spelling on macOS

The bundled Hunspell dictionary (~49k SCOWL roots) lacked common words like
"cybersecurity", "scalable", and "assistantship", and checked words inside
URLs. On macOS, spelling now uses the system NSSpellChecker (same engine as
TextEdit/Safari): far better vocabulary, skips URLs, and respects words the
user has taught macOS. The custom/tech dictionaries and Harper grammar are
unchanged; non-macOS builds keep the Hunspell fallback.

src-tauri/Cargo.lock

@@ -4713,11 +4713,14 @@ checksum = "3e2e65a1a2e43cfcb47a895c4c8b10d1f4a61097f9f254f183aee60cad9c651d"
 
 [[package]]
 name = "margin-app"
-version = "0.1.7"
+version = "0.1.9"
 dependencies = [
  "base64 0.22.1",
  "fontdb",
  "harper-core",
+ "objc2",
+ "objc2-app-kit",
+ "objc2-foundation",
  "rand 0.8.6",
  "reqwest 0.12.28",
  "serde",
@@ -5138,9 +5141,17 @@ checksum = "d49e936b501e5c5bf01fda3a9452ff86dc3ea98ad5f283e1455153142d97518c"
 dependencies = [
  "bitflags 2.13.0",
  "block2",
+ "libc",
  "objc2",
+ "objc2-cloud-kit",
+ "objc2-core-data",
  "objc2-core-foundation",
+ "objc2-core-graphics",
+ "objc2-core-image",
+ "objc2-core-text",
+ "objc2-core-video",
  "objc2-foundation",
+ "objc2-quartz-core",
 ]
 
 [[package]]
@@ -5160,6 +5171,7 @@ version = "0.3.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "0b402a653efbb5e82ce4df10683b6b28027616a2715e90009947d50b8dd298fa"
 dependencies = [
+ "bitflags 2.13.0",
  "objc2",
  "objc2-foundation",
 ]
@@ -5220,6 +5232,19 @@ dependencies = [
  "objc2-core-graphics",
 ]
 
+[[package]]
+name = "objc2-core-video"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d425caf1df73233f29fd8a5c3e5edbc30d2d4307870f802d18f00d83dc5141a6"
+dependencies = [
+ "bitflags 2.13.0",
+ "objc2",
+ "objc2-core-foundation",
+ "objc2-core-graphics",
+ "objc2-io-surface",
+]
+
 [[package]]
 name = "objc2-encode"
 version = "4.1.0"