fix(A.2 #13): propagate whisper feature from kon crate through to kon-transcription

Review feedback: src-tauri/src/commands/models.rs was still naming
load_whisper unconditionally, so a --no-default-features workspace
build (kon-transcription without the whisper feature) would have
compiled the transcription crate cleanly but failed in the kon crate
as soon as it tried to resolve the load_whisper symbol.

Adds a matching [features] section to src-tauri/Cargo.toml:
- default = ["whisper"]
- whisper = ["kon-transcription/whisper"]

and declares the kon-transcription dep with default-features = false so
the feature actually propagates rather than being forced-on by the
child crate's default. Cfg-gates the load_whisper import in models.rs
and adds a companion match arm that returns a runtime error when the
feature is off, keeping the API shape intact.

Verified with both cargo build -p kon and cargo build -p kon
--no-default-features.

src-tauri/src/commands/models.rs

@@ -9,7 +9,9 @@ use kon_core::hardware::{self, CpuFeatures};
 use kon_core::model_registry::{self, Engine, LanguageSupport, ModelEntry};
 use kon_core::types::{AudioSamples, ModelId, TranscriptionOptions};
 use kon_transcription::model_manager;
-use kon_transcription::{load_parakeet, load_whisper, LocalEngine, Transcriber};
+use kon_transcription::{load_parakeet, LocalEngine, Transcriber};
+#[cfg(feature = "whisper")]
+use kon_transcription::load_whisper;
 
 /// Map legacy size strings to ModelId.
 fn whisper_model_id(size: &str) -> ModelId {
@@ -78,6 +80,7 @@ pub fn load_model_from_disk(
         model_registry::find_model(model_id).ok_or_else(|| format!("Unknown model: {model_id}"))?;
 
     match entry.engine {
+        #[cfg(feature = "whisper")]
         Engine::Whisper => {
             let dir = model_manager::model_dir(model_id);
             let model_file = entry
@@ -90,6 +93,11 @@ pub fn load_model_from_disk(
             }
             load_whisper(&model_file).map_err(|e| e.to_string())
         }
+        #[cfg(not(feature = "whisper"))]
+        Engine::Whisper => Err(format!(
+            "Whisper backend not compiled in this build (kon built without the \"whisper\" feature); \
+             cannot load {model_id}"
+        )),
         Engine::Parakeet => {
             let dir = model_manager::model_dir(model_id);
             if !dir.exists() {