Faster large-note opens and smoother switching

Improved

• Large note performance — notes over 100k characters now highlight from the visible region first instead of blocking on the full document, cutting worst-case open time from ~1.3s to under 70ms
• Preview styling — softer table borders and lighter frontmatter key styling for a calmer reading view

Fixed

• Fast note switches no longer briefly flash the previous note's source text
• Reopening a note in source mode no longer causes a scroll jump
• Opening files from Finder into a cold app works reliably again