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/ai-formatting/src/pipeline.rs

@@ -76,7 +76,17 @@ pub fn post_process_segments(
                 .join(" ");
 
             if !joined.is_empty() {
-                match llm_client::cleanup_text(engine, &joined, &options.dictionary_terms) {
+                // Pipeline-internal cleanup (used by file-based + live
+                // transcribe paths) runs with the Default preset. The
+                // named-preset UX (B.1 #15) flows through the explicit
+                // cleanup_transcript_text_cmd path instead, where the
+                // frontend decides which preset the user has selected.
+                match llm_client::cleanup_text(
+                    engine,
+                    &joined,
+                    &options.dictionary_terms,
+                    llm_client::LlmPromptPreset::Default,
+                ) {
                     Ok(cleaned) if !cleaned.trim().is_empty() => {
                         replace_segments_with_cleaned(segments, cleaned.trim());
                     }