feat(ai B.1 #15 + A.1 #28): cleanup presets and sequential-GPU guard

Two new Settings → AI knobs that compose cleanly with what already
shipped (aiTier, LLM model, translator prompt framing).

**B.1 #15 — Named cleanup presets.** LlmPromptPreset enum
(Default / Email / Notes / Code) appends a short context hint onto
the CLEANUP_PROMPT just before generation. Presets shape tone and
structure ("email paragraph", "bulleted meeting notes", "preserve
technical terms") without licensing the content-editing the
translator-not-editor framing forbids. cleanup_transcript_text_cmd
now takes `preset: Option<String>` which runs through the new
LlmPromptPreset::parse (normalises aliases like "meeting-notes",
collapses unknown values to Default).

**A.1 #28 — Sequential-GPU guard.** New LocalEngine::unload drops
the backend + model_id so a subsequent load actually reclaims VRAM.
load_llm_model, load_model, and load_parakeet_model Tauri commands
grow an optional `concurrent: bool` argument. When concurrent is
Some(false), loading LLM first unloads whisper+parakeet, and vice
versa — prevents VRAM OOM on tight-VRAM setups. Default is the
previous parallel behaviour so nothing changes for multi-GB cards.
Transcribe-in-progress paths (transcribe_pcm, transcribe_file, live)
pass None, so mid-dictation model loads don't accidentally tear
down the LLM.

Settings UI (AI section):
- Cleanup preset segmented button + descriptive copy for each option.
- GPU concurrency segmented button with explicit trade-off text
  ("faster transitions vs fits in tight VRAM").

Frontend wiring:
- settings.llmPromptPreset flows from DictationPage's
  cleanupTranscriptIfEnabled into the Tauri command.
- settings.aiGpuConcurrency flows from both DictationPage (auto-load
  on record) and SettingsPage (manual load/unload buttons) as
  `concurrent: "parallel" === true` to the load commands.

Tests: three new preset cases in crates/ai-formatting/src/llm_client.rs
(parse aliases, suffix non-empty for non-default, default suffix
empty). All 139 existing lib tests still pass.

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

crates/transcription/src/local_engine.rs

@@ -56,6 +56,23 @@ impl LocalEngine {
         *id_guard = Some(model_id);
     }
 
+    /// Drop the loaded model and free its backing resources (GPU VRAM,
+    /// CPU memory, mmap'd GGML tensors). Used by the sequential-GPU
+    /// guard (brief item A.1 #28) so loading the LLM on a tight-VRAM
+    /// system first frees the transcription engine, and vice versa.
+    ///
+    /// No-op when nothing is loaded. Thread-safe — the internal Mutex
+    /// serialises against concurrent transcribe_sync calls.
+    pub fn unload(&self) {
+        let mut guard = self.engine.lock().unwrap_or_else(|e| e.into_inner());
+        *guard = None;
+        let mut id_guard = self
+            .loaded_model_id
+            .lock()
+            .unwrap_or_else(|e| e.into_inner());
+        *id_guard = None;
+    }
+
     pub fn name(&self) -> &EngineName {
         &self.engine_name
     }