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>

package.json

@@ -27,6 +27,32 @@
   },
   "license": "MIT",
   "dependencies": {
+    "@codemirror/commands": "6.8.1",
+    "@codemirror/lang-angular": "^0.1.4",
+    "@codemirror/lang-cpp": "^6.0.3",
+    "@codemirror/lang-css": "^6.3.1",
+    "@codemirror/lang-go": "^6.0.1",
+    "@codemirror/lang-html": "^6.4.11",
+    "@codemirror/lang-java": "^6.0.2",
+    "@codemirror/lang-javascript": "^6.2.4",
+    "@codemirror/lang-json": "^6.0.2",
+    "@codemirror/lang-less": "^6.0.2",
+    "@codemirror/lang-markdown": "^6.5.0",
+    "@codemirror/lang-php": "^6.0.2",
+    "@codemirror/lang-python": "^6.2.1",
+    "@codemirror/lang-rust": "^6.0.2",
+    "@codemirror/lang-sass": "^6.0.2",
+    "@codemirror/lang-sql": "^6.10.0",
+    "@codemirror/lang-vue": "^0.1.3",
+    "@codemirror/lang-wast": "^6.0.2",
+    "@codemirror/lang-xml": "^6.1.0",
+    "@codemirror/lang-yaml": "^6.1.2",
+    "@codemirror/language": "^6.12.1",
+    "@codemirror/legacy-modes": "^6.5.2",
+    "@codemirror/state": "^6.5.4",
+    "@codemirror/theme-one-dark": "^6.1.3",
+    "@codemirror/view": "^6.39.11",
+    "@lezer/highlight": "^1.2.3",
     "@tauri-apps/api": "^2",
     "@tauri-apps/plugin-clipboard-manager": "^2.3.2",
     "@tauri-apps/plugin-dialog": "^2",
@@ -36,6 +62,7 @@
     "@tauri-apps/plugin-os": "^2",
     "@tauri-apps/plugin-shell": "^2.3.4",
     "@tauri-apps/plugin-store": "^2",
+    "codemirror": "^6.0.2",
     "highlight.js": "^11.11.1",
     "marked": "^17.0.1"
   },