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>

src/lib/pages/DictationPage.svelte

@@ -239,7 +239,13 @@
     try {
       const status = await invoke("check_llm_model", { modelId: settings.llmModelId });
       if (status?.downloaded && !status.loaded) {
-        await invoke("load_llm_model", { modelId: settings.llmModelId });
+        await invoke("load_llm_model", {
+          modelId: settings.llmModelId,
+          // Sequential-GPU guard (brief item A.1 #28): frees whisper
+          // before loading the LLM on tight-VRAM setups. "parallel"
+          // keeps both resident (default, safe on multi-GB cards).
+          concurrent: settings.aiGpuConcurrency === "parallel",
+        });
       }
     } catch (err) {
       console.warn("ensureLlmModelLoaded failed", err);
@@ -257,10 +263,15 @@
     error = "";
 
     try {
+      // concurrent flag mirrors settings.aiGpuConcurrency (brief A.1 #28).
+      const concurrent = settings.aiGpuConcurrency === "parallel";
       if (settings.engine === "parakeet") {
-        await invoke("load_parakeet_model", { name: "ctc-int8" });
+        await invoke("load_parakeet_model", { name: "ctc-int8", concurrent });
       } else {
-        await invoke("load_model", { size: settings.modelSize.toLowerCase() });
+        await invoke("load_model", {
+          size: settings.modelSize.toLowerCase(),
+          concurrent,
+        });
       }
       await refreshRuntimeCapabilities();
       modelReady = true;
@@ -460,6 +471,7 @@
       const cleaned = await invoke("cleanup_transcript_text_cmd", {
         transcript: text,
         profileId: profilesStore.activeProfileId,
+        preset: settings.llmPromptPreset,
       });
       markGenerationDone(true);
       return cleaned?.trim() ? cleaned.trim() : text;