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/InputBar.svelte

@@ -30,6 +30,7 @@
   import SnippetLibraryPanel from "$lib/components/SnippetLibraryPanel.svelte";
   import QuickActionsPanel from "$lib/components/QuickActionsPanel.svelte";
   import ClipboardHistoryPanel from "$lib/components/ClipboardHistoryPanel.svelte";
+  import TextInputContextMenu from "$lib/components/TextInputContextMenu.svelte";
   import type { Attachment } from "$lib/types/messages";
 
   const INPUT_HISTORY_KEY = "hikari-input-history";
@@ -49,6 +50,23 @@
   let showClipboardHistory = $state(false);
   let streamerModeActive = $state(false);
 
+  // Context menu state
+  let textareaElement: HTMLTextAreaElement | undefined = $state();
+  let contextMenuShow = $state(false);
+  let contextMenuX = $state(0);
+  let contextMenuY = $state(0);
+
+  function handleContextMenu(event: MouseEvent) {
+    event.preventDefault();
+    contextMenuShow = true;
+    contextMenuX = event.clientX;
+    contextMenuY = event.clientY;
+  }
+
+  function closeContextMenu() {
+    contextMenuShow = false;
+  }
+
   isStreamerMode.subscribe((value) => {
     streamerModeActive = value;
   });
@@ -876,10 +894,12 @@ User: ${formattedMessage}`;
       <!-- svelte-ignore a11y_no_static_element_interactions -->
       <div class="resize-handle" onmousedown={handleResizeStart} title="Drag to resize"></div>
       <textarea
+        bind:this={textareaElement}
         bind:value={inputValue}
         onkeydown={handleKeyDown}
         oninput={handleInputChange}
         onpaste={handlePaste}
+        oncontextmenu={handleContextMenu}
         placeholder={isConnected
           ? "Ask Hikari anything... (type / for commands)"
           : "Connect to Claude first..."}
@@ -958,6 +978,15 @@ User: ${formattedMessage}`;
   />
 {/if}
 
+{#if contextMenuShow && textareaElement}
+  <TextInputContextMenu
+    x={contextMenuX}
+    y={contextMenuY}
+    inputElement={textareaElement}
+    onClose={closeContextMenu}
+  />
+{/if}
+
 <style>
   .input-bar {
     display: flex;