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/editor/ConfirmDialog.svelte

@@ -0,0 +1,131 @@
+<script lang="ts">
+  interface Props {
+    title: string;
+    message: string;
+    confirmText?: string;
+    cancelText?: string;
+    destructive?: boolean;
+    onConfirm: () => void;
+    onCancel: () => void;
+  }
+
+  let {
+    title,
+    message,
+    confirmText = "Confirm",
+    cancelText = "Cancel",
+    destructive = false,
+    onConfirm,
+    onCancel,
+  }: Props = $props();
+
+  function handleKeydown(event: KeyboardEvent) {
+    if (event.key === "Escape") {
+      onCancel();
+    } else if (event.key === "Enter") {
+      onConfirm();
+    }
+  }
+</script>
+
+<svelte:window onkeydown={handleKeydown} />
+
+<!-- svelte-ignore a11y_no_static_element_interactions -->
+<div class="dialog-overlay" onclick={onCancel}>
+  <!-- svelte-ignore a11y_no_static_element_interactions -->
+  <div class="dialog-content" onclick={(e) => e.stopPropagation()}>
+    <h2 class="dialog-title">{title}</h2>
+    <p class="dialog-message">{message}</p>
+
+    <div class="dialog-actions">
+      <button class="btn-cancel" onclick={onCancel}>
+        {cancelText}
+      </button>
+      <button class="btn-confirm" class:destructive onclick={onConfirm}>
+        {confirmText}
+      </button>
+    </div>
+  </div>
+</div>
+
+<style>
+  .dialog-overlay {
+    position: fixed;
+    inset: 0;
+    z-index: 50;
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    background-color: rgba(0, 0, 0, 0.5);
+  }
+
+  .dialog-content {
+    margin: 0 1rem;
+    width: 100%;
+    max-width: 28rem;
+    border-radius: 0.5rem;
+    border: 1px solid var(--border-color);
+    background-color: var(--bg-secondary);
+    padding: 1.5rem;
+    box-shadow:
+      0 20px 25px -5px rgba(0, 0, 0, 0.1),
+      0 10px 10px -5px rgba(0, 0, 0, 0.04);
+  }
+
+  .dialog-title {
+    margin-bottom: 0.5rem;
+    font-size: 1.125rem;
+    font-weight: 600;
+    color: var(--text-primary);
+  }
+
+  .dialog-message {
+    margin-bottom: 1.5rem;
+    font-size: 0.875rem;
+    color: var(--text-secondary);
+  }
+
+  .dialog-actions {
+    display: flex;
+    justify-content: flex-end;
+    gap: 0.75rem;
+  }
+
+  .btn-cancel {
+    border-radius: 0.375rem;
+    border: 1px solid var(--border-color);
+    background-color: var(--bg-primary);
+    padding: 0.5rem 1rem;
+    font-size: 0.875rem;
+    color: var(--text-primary);
+    cursor: pointer;
+    transition: background-color 0.15s ease;
+  }
+
+  .btn-cancel:hover {
+    background-color: var(--bg-secondary);
+  }
+
+  .btn-confirm {
+    border-radius: 0.375rem;
+    border: none;
+    background-color: var(--accent-primary);
+    padding: 0.5rem 1rem;
+    font-size: 0.875rem;
+    color: white;
+    cursor: pointer;
+    transition: background-color 0.15s ease;
+  }
+
+  .btn-confirm:hover {
+    background-color: var(--accent-secondary);
+  }
+
+  .btn-confirm.destructive {
+    background-color: #dc2626;
+  }
+
+  .btn-confirm.destructive:hover {
+    background-color: #b91c1c;
+  }
+</style>