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/lib/components/KeyboardShortcutsModal.svelte

@@ -12,6 +12,7 @@
         { keys: ["Escape"], description: "Close modals and panels" },
         { keys: ["Ctrl", "L"], description: "Clear the terminal" },
         { keys: ["Ctrl", ","], description: "Open settings" },
+        { keys: ["Ctrl", "E"], description: "Toggle file editor" },
         { keys: ["Ctrl", "Shift", "M"], description: "Toggle compact mode" },
         { keys: ["Ctrl", "Shift", "S"], description: "Toggle streamer mode" },
       ],
@@ -26,6 +27,17 @@
         { keys: ["↓"], description: "Next input from history" },
       ],
     },
+    {
+      category: "File Editor",
+      items: [
+        { keys: ["Ctrl", "E"], description: "Toggle editor view" },
+        { keys: ["Ctrl", "B"], description: "Toggle file browser" },
+        { keys: ["Ctrl", "S"], description: "Save current file" },
+        { keys: ["Ctrl", "W"], description: "Close current tab" },
+        { keys: ["Ctrl", "N"], description: "New file" },
+        { keys: ["Right-click"], description: "Context menu (New/Delete)" },
+      ],
+    },
     {
       category: "Slash Commands",
       items: [