Add "Preview mode" on file editor for Markdown files

[KiKaraage]

I think it's convenient to be able to read Markdown files comfortably, especially when those are generated by agents. It's nice to read table as an.. actual table on those md files.

Proof of Concept

What I implemented on my Shelley fork:

• Adding a Preview mode toggle besides Comment mode and Edit mode, toggleable via Ctrl+Shift+K when EditableFileModal is opened. Toggle only shown when the opened file explicitly has .md extension
• Reuse MarkdownContent.vue component, make it centered against the modal for viewing comfort (.diff-viewer-preview - scrollable, padded, prose max-width 72ch, centered via margin: 0 auto)
• Add basic Vim navigation support (j/k scroll by line, gg/G top/bottom, Ctrl+d/u/f/b page jumps, i/a/o return to edit mode and focus the editor) so it's still usable when Vim mode is enabled
• Files edited: ui/src/vue/components/EditableFileModal.vue, ui/src/vue/composables/monacoComments.ts, ui/src/styles.css

Light Mode screenshot

[philz]

Do you want to open a PR and sign the CLA? If you prefer we re-implement, that's fine too.

[KiKaraage]

Do you want to open a PR and sign the CLA? If you prefer we re-implement, that's fine too.

Oh yeah sure!