Implement Live View mode

[ZFordDev]

Description

Live View is a planned preview mode that shows a real-time preview of the markdown alongside the editor, without requiring a toggle. Currently:

• index.html:112-116 — button exists with .disabled class and "soon" badge
• viewMode.js:149-156 — applyLiveMode() silently falls back to preview mode
• viewMode.js:13 — JSDoc documents it as "disabled, future"

Expected Behavior

When Live View is active:

• Preview updates automatically on every keystroke (debounced)
• Editor and preview are visible simultaneously (similar to split view)
• No manual toggle required — preview is always live
• Mode persists across sessions via localStorage

Notes

This feature needs to be implemented alongside the Tauri migration in the Phoenix branch. The Electron version can remain as-is until Phoenix is ready.

[ZFordDev]

Status update:

This is proving more difficult than I initially anticipated. While implementing it with out consequence is easy I'm not prepared to increase the active ram footprint that much.

Open to suggestions on this one else we might drop the feature.