File-based project save/load, AI chat formatting, text edit fix

Project files (.vtn):
- Save Project: serializes transcript, speakers, audio path to JSON file
- Open Project: loads .vtn file, restores audio/transcript/speakers
- User chooses filename and location via save dialog
- Replaces SQLite-based project persistence (DB commands remain for future use)
- Text edits update in-memory store immediately, persist on explicit save
- Fix Windows path separator in project name extraction

AI chat:
- Markdown rendering in assistant messages (headers, lists, bold, code)
- Better visual distinction with border-left accents
- Styled markdown elements for dark theme

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -10,8 +10,8 @@ use tauri::Manager;
 use commands::ai::{ai_chat, ai_configure, ai_list_providers};
 use commands::export::export_transcript;
 use commands::project::{
-    create_project, delete_project, get_project, list_projects, load_project_transcript,
-    save_project_transcript, update_segment,
+    create_project, delete_project, get_project, list_projects, load_project_file,
+    load_project_transcript, save_project_file, save_project_transcript, update_segment,
 };
 use commands::settings::{load_settings, save_settings};
 use commands::system::{get_data_dir, llama_list_models, llama_start, llama_status, llama_stop};
@@ -41,6 +41,8 @@ pub fn run() {
             save_project_transcript,
             load_project_transcript,
             update_segment,
+            save_project_file,
+            load_project_file,
             transcribe_file,
             run_pipeline,
             download_diarize_model,