feat(whisper): feed profile_terms into initial_prompt at decode time

Previously profile_terms only reached the LLM cleanup stage as the
dictionary_terms suffix. Whisper decoded without any vocabulary hint, so
domain names ('Wren', 'CORBEL') were misspelled on the first pass and the
LLM had to guess at the correction.

build_initial_prompt (src-tauri/src/commands/mod.rs) collapses caller /
profile / terms into a single Whisper prompt:
  caller_prompt > profile_prompt + "Vocabulary: <terms>." > None

transcribe_pcm, transcribe_file, and start_live_transcription_session all
route through the helper, so the three paths stay in lockstep.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src-tauri/src/commands/mod.rs

@@ -12,3 +12,93 @@ pub mod transcription;
 pub mod transcripts;
 pub mod update;
 pub mod windows;
+
+/// Build the Whisper `initial_prompt` for a transcription request.
+///
+/// Precedence:
+/// 1. Caller-supplied `request_prompt` (non-empty wins outright — the caller
+///    has already made the decision).
+/// 2. Profile's stored prompt + profile terms (joined: the prompt frames the
+///    task, the vocabulary biases recognition toward domain terms).
+/// 3. Profile prompt alone, or vocabulary alone.
+/// 4. `None` if nothing is set.
+///
+/// Feeding `profile_terms` into `initial_prompt` (the OpenWhispr pattern) lets
+/// whisper.cpp bias its decoder toward the correct spelling of user-specific
+/// vocabulary at decode time, before any LLM cleanup pass.
+pub fn build_initial_prompt(
+    request_prompt: &str,
+    profile_prompt: &str,
+    profile_terms: &[String],
+) -> Option<String> {
+    let trimmed_request = request_prompt.trim();
+    if !trimmed_request.is_empty() {
+        return Some(trimmed_request.to_string());
+    }
+
+    let trimmed_profile = profile_prompt.trim();
+    let terms_list = profile_terms
+        .iter()
+        .map(|term| term.trim())
+        .filter(|term| !term.is_empty())
+        .collect::<Vec<_>>()
+        .join(", ");
+
+    match (trimmed_profile.is_empty(), terms_list.is_empty()) {
+        (true, true) => None,
+        (false, true) => Some(trimmed_profile.to_string()),
+        (true, false) => Some(format!("Vocabulary: {terms_list}.")),
+        (false, false) => Some(format!("{trimmed_profile} Vocabulary: {terms_list}.")),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::build_initial_prompt;
+
+    #[test]
+    fn caller_prompt_overrides_everything() {
+        let got = build_initial_prompt(
+            "caller wins",
+            "profile prompt",
+            &["Wren".into(), "CORBEL".into()],
+        );
+        assert_eq!(got.as_deref(), Some("caller wins"));
+    }
+
+    #[test]
+    fn profile_prompt_and_terms_are_joined() {
+        let got = build_initial_prompt(
+            "",
+            "You are a meeting notes assistant.",
+            &["Wren".into(), "CORBEL".into()],
+        );
+        assert_eq!(
+            got.as_deref(),
+            Some("You are a meeting notes assistant. Vocabulary: Wren, CORBEL."),
+        );
+    }
+
+    #[test]
+    fn terms_only_produces_vocabulary_sentence() {
+        let got = build_initial_prompt("", "", &["Wren".into(), "CORBEL".into()]);
+        assert_eq!(got.as_deref(), Some("Vocabulary: Wren, CORBEL."));
+    }
+
+    #[test]
+    fn profile_prompt_alone_is_passed_through() {
+        let got = build_initial_prompt("", "Be concise.", &[]);
+        assert_eq!(got.as_deref(), Some("Be concise."));
+    }
+
+    #[test]
+    fn all_empty_returns_none() {
+        assert_eq!(build_initial_prompt("", "", &[]), None);
+    }
+
+    #[test]
+    fn whitespace_only_terms_are_skipped() {
+        let got = build_initial_prompt("", "", &["  ".into(), "Wren".into(), "".into()]);
+        assert_eq!(got.as_deref(), Some("Vocabulary: Wren."));
+    }
+}