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/types/app.ts

@@ -12,6 +12,8 @@ export type WhisperModelSize =
   | "Distil-L";
 export type AiTier = "off" | "cleanup" | "tasks";
 export type LlmModelIdStr = "qwen3_1_7b" | "qwen3_4b_instruct_2507" | "qwen3_14b";
+export type LlmPromptPreset = "default" | "email" | "notes" | "code";
+export type AiGpuConcurrency = "parallel" | "sequential";
 export type TaskBucket = "inbox" | "today" | "soon" | "later";
 export type ToastSeverity = "info" | "success" | "warn" | "error";
 
@@ -47,6 +49,8 @@ export interface SettingsState {
   fontSize: number;
   aiTier: AiTier;
   llmModelId: LlmModelIdStr | null;
+  llmPromptPreset: LlmPromptPreset;
+  aiGpuConcurrency: AiGpuConcurrency;
   saveAudio: boolean;
   outputFolder: string;
   globalHotkey: string;