UI polish: scrollbar, focus ring, active nav state, transitions, sidebar border

[CTLBs]

Summary

Small, non-breaking visual improvements to the dark-theme UI. The spirit of the existing design is fully preserved.

• Scrollbar - width increased from 1px to 6px (h-1.5 w-1.5) for better usability
• Focus accessibility - replaced the global *:focus { outline: none } override with *:focus-visible, so keyboard users get a clear hl-500 outline while mouse users see nothing
• Input fields - added a visible border (border-[var(--bg-600)]) with a focus-shift to hl-600, making form fields easier to identify
• Transitions - bumped from 100ms to 150ms across _focusable-base and buttons for a smoother feel
• Buttons - border-radius changed from rounded-md to rounded-lg for consistency with nav items
• Sidebar - added a subtle border-r border-[var(--bg-600)] to visually separate the drawer from the main content area
• Active nav item - was bg-[var(--hl-900)] (barely visible); now bg-[var(--hl-800)] with a border-l-2 border-[var(--hl-400)] left accent for clear wayfinding
• Logo - added text-lg tracking-wide and brightened the ai accent from hl-500 to hl-400

Files changed

• web/tailwind.css - focus-visible, scrollbar, transitions, input border, button radius
• web/app.css - sidebar border, active nav state
• web/Navigation.tsx - logo typography

Test plan

• Verify sidebar renders with right border on desktop (>=1280px)
• Click nav links and confirm active item shows blue left accent
• Tab through the UI and confirm focus ring appears on interactive elements
• Check input fields show border in forms (presets, character editor, etc.)
• Confirm buttons look correct across all schema variants (primary, red, green, secondary)
• Check mobile drawer still slides in/out correctly

[sceuick]

Thanks for the contribution. I've rolled some of these changes into #1305.