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/StatusBar.svelte

@@ -13,6 +13,7 @@
   import { get } from "svelte/store";
   import { claudeStore } from "$lib/stores/claude";
   import { configStore, type HikariConfig, isStreamerMode } from "$lib/stores/config";
+  import { editorStore } from "$lib/stores/editor";
   import type { ConnectionStatus } from "$lib/types/messages";
   import { onMount } from "svelte";
   import StatsDisplay from "./StatsDisplay.svelte";
@@ -80,6 +81,15 @@
     streamerModeActive = value;
   });
 
+  let editorVisible = $state(false);
+  editorStore.isEditorVisible.subscribe((value) => {
+    editorVisible = value;
+  });
+
+  function toggleEditor() {
+    editorStore.toggleEditor();
+  }
+
   onMount(async () => {
     appVersion = await getVersion();
   });
@@ -307,6 +317,25 @@
         />
       </svg>
     </button>
+    <button
+      onclick={toggleEditor}
+      disabled={connectionStatus !== "connected"}
+      class="p-1 text-gray-500 icon-trans-hover {editorVisible
+        ? 'text-[var(--trans-pink)]'
+        : ''} disabled:opacity-40 disabled:cursor-not-allowed"
+      title={connectionStatus === "connected"
+        ? "File Editor (Ctrl+E)"
+        : "Connect to enable file editor"}
+    >
+      <svg class="w-5 h-5" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+        <path
+          stroke-linecap="round"
+          stroke-linejoin="round"
+          stroke-width="2"
+          d="M11 5H6a2 2 0 00-2 2v11a2 2 0 002 2h11a2 2 0 002-2v-5m-1.414-9.414a2 2 0 112.828 2.828L11.828 15H9v-2.828l8.586-8.586z"
+        />
+      </svg>
+    </button>
     <button
       onclick={() => (showStats = !showStats)}
       class="p-1 text-gray-500 icon-trans-hover {showStats ? 'text-[var(--trans-pink)]' : ''}"