feat(editor): add Open file button to canvas tab bar

[alogotron]

Problem

When the Editor runs as a canvas surface (panel mode), the + button in the tab bar is hardcoded to create a new Markdown file only:

@click="$store.editor.runNewMenuAction('markdown')"

The "Open" option (file browser) is only available:

• In the empty state screen (no files open)
• In the modal-mode header dropdown (installHeaderNewMenu), which requires .modal-inner / .modal-header elements that are not present in canvas panel mode

This means once any file is open, there is no accessible UI to open a second existing file without first closing all tabs.

Fix

Adds a folder_open icon button immediately before the + (New Markdown) button in the tab bar.

It calls the existing runNewMenuAction('open') → openFileBrowser() pipeline — no logic changes required.

<button type="button" class="editor-new-tab"
  title="Open file" aria-label="Open file"
  @click="$store.editor.runNewMenuAction('open')">
  <span class="material-symbols-outlined">folder_open</span>
</button>

Result

Users can now open an existing file at any time directly from the tab bar, without closing the current file(s).

Change scope

• 1 file changed: plugins/_editor/webui/editor-panel.html
• +10 lines — one new button element
• No logic, store, API, or style changes