feat(web): markdown Source | Preview toggle in session file pane (#957)

* feat(web): markdown Source | Preview toggle in session file pane

Add Source | Preview toggle for .md/.mdx files in the session file route,
defaulting to preview with localStorage persistence. Reuse chat markdown
pipeline via MarkdownRenderer standalone mode (no assistant-ui thread).

Includes unit tests, Playwright smoke, and e2e fixture.

Closes tiann/hapi#954

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(web): cast standalone MarkdownRenderer components for react-markdown

Soup verify gate: defaultComponents merge type is wider than
react-markdown Components; standalone file-pane path needs explicit cast.

* fix(web): route file-pane markdown fences through SyntaxHighlighter

Standalone file preview now mirrors chat code-block rendering: fenced
blocks use SyntaxHighlighter and MARKDOWN_COMPONENTS_BY_LANGUAGE (mermaid
included) without requiring ThreadPrimitive context.

Addresses HAPI Bot Major on tiann/hapi#957.

Co-authored-by: Cursor <cursoragent@cursor.com>

* fix(web): detect fenced vs inline code in standalone markdown preview

Move block detection to the pre override (react-markdown v10 does not pass
inline to custom code components). Add inline-code regression test.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

web/src/lib/locales/en.ts

@@ -336,6 +336,8 @@ export default {
   'file.page.download': 'Download file',
   'file.page.tab.diff': 'Diff',
   'file.page.tab.file': 'File',
+  'file.page.tab.source': 'Source',
+  'file.page.tab.preview': 'Preview',
   'file.page.missingPath': 'No file path provided.',
   'file.page.binary': 'This looks like a binary file. It cannot be displayed.',
   'file.page.imagePreviewAlt': 'Image preview for {name}',