feat(web): add download button to file viewer (#926)

Adds a download icon button to the file viewer toolbar (next to copy-path).
Clicking it decodes the existing base64 file content into a Blob and triggers
a browser download — no new backend endpoint required. Works for text, binary,
and image files. Button is hidden until the file has loaded successfully.

Closes #924

via [HAPI](https://hapi.run)

Co-authored-by: HAPI <noreply@hapi.run>

web/src/lib/locales/en.ts

@@ -310,6 +310,7 @@ export default {
   'file.page.unknownPath': 'Unknown path',
   'file.page.copyPath': 'Copy path',
   'file.page.copyContent': 'Copy file content',
+  'file.page.download': 'Download file',
   'file.page.tab.diff': 'Diff',
   'file.page.tab.file': 'File',
   'file.page.missingPath': 'No file path provided.',