fix: fluid UI scaling for smaller displays

[nimish-ks]

Summary

• Adds a CSS clamp() on the root font-size that scales the entire UI from 14px (at 1024px viewport) to 16px (at 1920px viewport). Since all sizing uses rem via Tailwind, this proportionally scales fonts, buttons, padding, sidebar, navbar — everything.
• Fixes hardcoded px values (pt-[64px], calc(100vh-64px)) that caused layout misalignment when the root font-size changes, replacing them with their rem equivalents (pt-16, calc(100vh-4rem)).
• 27" 2K displays are completely unaffected (16px at 1920px). MacBook 13-15" screens get a ~6-12% proportional reduction.

Viewport	Effective font-size
1024px (tablet)	14px
1440px (MacBook Air 13")	~14.9px
1512px (MacBook Pro 14")	~15.1px
1728px (MacBook Pro 16")	~15.6px
1920px (27" 2K)	16px

Test plan

• Test on a 27" 2K display — should look identical to current
• Test on a 13-15" MacBook — UI should feel more proportionate
• Verify navbar and sidebar alignment (no gaps between navbar and sidebar/content)
• Check that toast notifications, dialogs, and dropdowns still render correctly
• Resize browser window from 1920px down to 1024px and verify smooth scaling

🤖 Generated with Claude Code

[nimish-ks]

@rohan-chaturvedi This is a possible alternative to #668 which doesn't break the UI.

LMK thoughts