Give Markdown and HTML file tabs back their Preview and Source views (#7614)

* fix(app): give Markdown and HTML file tabs back their Preview and Source views

The built-in Markdown and HTML panels own one toolbar again: file size,
line count, save status, vim mode, cursor, and a Preview | Source control
over a body that shows the rendered preview or the editor from the same
loaded content and editor model. Plain text files keep the toolbar on top.

The host no longer renders a panel switcher. Source is the built-in panel
for text files no other built-in claims, so it is never offered beside
Markdown or HTML in the picker, the Open with menu, or Settings > Files.
A line target opens the file's built-in panel in its Source view.

* refactor(app): name the shared text file panel TextFilePanel

Markdown and HTML own a Source view; they are not Source panels. The
composite in file-panels/source/panel.tsx is now TextFilePanel, and
SourcePanel is TextFilePanel without a Preview. Names only.

plugin-examples/markdown-panel/README.md

@@ -15,8 +15,9 @@ The panel is a CodeMirror editor with Markdown highlighting, line numbers, undo,
 wrapping. It reads the file once with `readText`, saves 500 ms after the last edit with
 `writeText`, and reports unsaved edits with `tab.setModified`. When the file changes on disk with
 nothing unsaved, it reads again; with unsaved edits, or when a write rejects with
-`PluginFileConflictError`, it offers **Reload** and **Overwrite**. The bar's switcher still offers
-Paseo's Markdown and Source panels. Native shows selectable, read-only text through `useFileText`.
+`PluginFileConflictError`, it offers **Reload** and **Overwrite**. **Open with…** in the tab menu
+switches the tab back to Paseo's Markdown panel. Native shows selectable, read-only text through
+`useFileText`.
 
 Paseo bundles the editor dependencies into this plugin. They are loaded only on web; the native
 path uses React Native primitives. DOM integration lives in `client/web.ts`.