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/EditorTabs.svelte

@@ -0,0 +1,170 @@
+<script lang="ts">
+  import { editorStore } from "$lib/stores/editor";
+
+  const tabs = editorStore.tabs;
+  const activeTabId = editorStore.activeTabId;
+
+  function handleTabClick(tabId: string) {
+    editorStore.setActiveTab(tabId);
+  }
+
+  function handleCloseTab(event: MouseEvent, tabId: string) {
+    event.stopPropagation();
+    editorStore.closeTab(tabId);
+  }
+
+  function handleKeydown(event: KeyboardEvent, tabId: string) {
+    if (event.key === "Enter" || event.key === " ") {
+      event.preventDefault();
+      editorStore.setActiveTab(tabId);
+    }
+  }
+
+  function handleCloseKeydown(event: KeyboardEvent, tabId: string) {
+    if (event.key === "Enter" || event.key === " ") {
+      event.preventDefault();
+      event.stopPropagation();
+      editorStore.closeTab(tabId);
+    }
+  }
+</script>
+
+<div class="editor-tabs">
+  {#if $tabs.length === 0}
+    <div class="no-tabs">No files open</div>
+  {:else}
+    <div class="tabs-container" role="tablist">
+      {#each $tabs as tab (tab.id)}
+        <div
+          class="tab"
+          class:active={tab.id === $activeTabId}
+          class:dirty={tab.isDirty}
+          role="tab"
+          tabindex="0"
+          aria-selected={tab.id === $activeTabId}
+          on:click={() => handleTabClick(tab.id)}
+          on:keydown={(e) => handleKeydown(e, tab.id)}
+          title={tab.filePath}
+        >
+          <span class="tab-name">
+            {tab.fileName}
+            {#if tab.isDirty}
+              <span class="dirty-indicator">*</span>
+            {/if}
+          </span>
+          <button
+            class="close-button"
+            on:click={(e) => handleCloseTab(e, tab.id)}
+            on:keydown={(e) => handleCloseKeydown(e, tab.id)}
+            title="Close tab"
+            aria-label="Close {tab.fileName}"
+          >
+            <svg
+              width="12"
+              height="12"
+              viewBox="0 0 24 24"
+              fill="none"
+              stroke="currentColor"
+              stroke-width="2"
+            >
+              <path d="M18 6L6 18M6 6l12 12" />
+            </svg>
+          </button>
+        </div>
+      {/each}
+    </div>
+  {/if}
+</div>
+
+<style>
+  .editor-tabs {
+    display: flex;
+    align-items: center;
+    background-color: var(--bg-secondary);
+    border-bottom: 1px solid var(--border-color);
+    min-height: 36px;
+  }
+
+  .no-tabs {
+    padding: 8px 12px;
+    color: var(--text-secondary);
+    font-size: 13px;
+  }
+
+  .tabs-container {
+    display: flex;
+    overflow-x: auto;
+    scrollbar-width: thin;
+  }
+
+  .tab {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 8px 12px;
+    background: transparent;
+    border: none;
+    border-right: 1px solid var(--border-color);
+    color: var(--text-secondary);
+    font-size: 13px;
+    cursor: pointer;
+    white-space: nowrap;
+    transition: all 0.15s ease;
+  }
+
+  .tab:hover {
+    background-color: var(--bg-primary);
+    color: var(--text-primary);
+  }
+
+  .tab:focus {
+    outline: 1px solid var(--accent-primary);
+    outline-offset: -1px;
+  }
+
+  .tab.active {
+    background-color: var(--bg-terminal);
+    color: var(--text-primary);
+    border-bottom: 2px solid var(--accent-primary);
+    margin-bottom: -1px;
+  }
+
+  .tab-name {
+    display: flex;
+    align-items: center;
+    gap: 2px;
+  }
+
+  .dirty-indicator {
+    color: var(--accent-primary);
+    font-weight: bold;
+  }
+
+  .close-button {
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    padding: 2px;
+    background: transparent;
+    border: none;
+    color: var(--text-secondary);
+    cursor: pointer;
+    border-radius: 4px;
+    opacity: 0;
+    transition: all 0.15s ease;
+  }
+
+  .tab:hover .close-button,
+  .tab.active .close-button {
+    opacity: 1;
+  }
+
+  .close-button:hover {
+    background-color: var(--bg-secondary);
+    color: var(--text-primary);
+  }
+
+  .tab.dirty .close-button {
+    opacity: 1;
+  }
+</style>