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/Cargo.toml

@@ -9,6 +9,15 @@ edition = "2021"
 name = "kon_lib"
 crate-type = ["staticlib", "cdylib", "rlib"]
 
+[features]
+# Default build includes the Whisper backend. Disabling this feature
+# also drops it from kon-transcription (see Cargo.toml in that crate)
+# so a --no-default-features workspace build does not pull whisper-rs-sys.
+# load_model_from_disk returns a runtime error for Engine::Whisper when
+# this feature is off; Parakeet continues to work.
+default = ["whisper"]
+whisper = ["kon-transcription/whisper"]
+
 [build-dependencies]
 tauri-build = { version = "2", features = [] }
 # Used by the CSP regression guard in build.rs to parse tauri.conf.json
@@ -21,7 +30,7 @@ serde_json = "1"
 # Workspace crates
 kon-core = { path = "../crates/core" }
 kon-audio = { path = "../crates/audio" }
-kon-transcription = { path = "../crates/transcription" }
+kon-transcription = { path = "../crates/transcription", default-features = false }
 kon-ai-formatting = { path = "../crates/ai-formatting" }
 kon-storage = { path = "../crates/storage" }
 kon-cloud-providers = { path = "../crates/cloud-providers" }