feat: add built-in file editor with syntax highlighting (#79)

## Summary
- Add CodeMirror 6 editor with syntax highlighting for 40+ languages
- Add file browser sidebar with collapsible directory tree navigation
- Add multi-tab support with dirty state indicators and close buttons
- Add keyboard shortcuts (Ctrl+E toggle, Ctrl+B file browser, Ctrl+S save, Ctrl+W close tab)
- Add editor toggle button to status bar (disabled when not connected)
- Editor automatically uses current session's working directory
- Add Tauri backend commands for file operations (list_directory, read_file_content, write_file_content)

## Test Plan
- [ ] Connect to a session and verify the editor toggle button becomes enabled
- [ ] Press Ctrl+E to open the editor and verify file tree shows the session's CWD
- [ ] Navigate directories and open files to verify syntax highlighting works
- [ ] Edit a file and verify the dirty indicator (*) appears
- [ ] Save with Ctrl+S and verify the dirty indicator disappears
- [ ] Open multiple files and verify tab switching works
- [ ] Close tabs with Ctrl+W or the X button
- [ ] Disconnect and verify the editor automatically closes
- [ ] Verify keyboard shortcuts are documented in the shortcuts modal

Closes #72

✨ This PR was created with help from Hikari~ 🌸

Reviewed-on: #79
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src/routes/+layout.svelte

@@ -1,9 +1,17 @@
-<script>
+<script lang="ts">
   import "../app.css";
 
   let { children } = $props();
+
+  // Prevent the default context menu globally
+  // Individual components can show their own context menus by calling event.stopPropagation()
+  function handleContextMenu(event: MouseEvent) {
+    event.preventDefault();
+  }
 </script>
 
+<svelte:window oncontextmenu={handleContextMenu} />
+
 <div id="app">
   {@render children()}
 </div>