Add auto-scroll, file dialog, and transcript editing

- Auto-scroll transcript to active segment during playback with smart
  pause when user manually scrolls (resumes after 3s)
- Replace prompt() with native Tauri file dialog for audio/video import
  with file type filters
- Add inline transcript editing via double-click with Enter to save,
  Esc to cancel, preserving original text for change tracking
- Show "edited" badge on modified segments

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/routes/+page.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import { invoke } from '@tauri-apps/api/core';
+  import { open } from '@tauri-apps/plugin-dialog';
   import WaveformPlayer from '$lib/components/WaveformPlayer.svelte';
   import TranscriptEditor from '$lib/components/TranscriptEditor.svelte';
   import SpeakerManager from '$lib/components/SpeakerManager.svelte';
@@ -20,8 +21,14 @@
   }
 
   async function handleFileImport() {
-    // For now, use a simple prompt — will be replaced with Tauri file dialog
-    const filePath = prompt('Enter path to audio/video file:');
+    const filePath = await open({
+      multiple: false,
+      filters: [{
+        name: 'Audio/Video',
+        extensions: ['mp3', 'wav', 'flac', 'ogg', 'm4a', 'aac', 'wma',
+                     'mp4', 'mkv', 'avi', 'mov', 'webm'],
+      }],
+    });
     if (!filePath) return;
 
     // Convert file path to URL for wavesurfer