Replace hardcoded light-mode Tailwind colors with dark: variants
across six components. Issues that broke readability:
- PiiDiffViewer / Security: toggle knob was bg-white (invisible on
bg-muted in dark mode) -> bg-background
- ImageGallery: thumbnail container, filename labels, alert banners,
and modal chrome all used hardcoded gray/white backgrounds with dark
text; added full dark: variants throughout
- ShellExecution TIER_CONFIG: tier cards used bg-green/yellow/red-50
(near-white) with dark text; added dark:bg-*-950/30 backgrounds and
light text for all three tiers
- ShellApprovalModal: tier 2 badge hardcoded bg-yellow-50/text-yellow-700;
added dark: variants
- LogUpload: PII warning alert used bg-amber-50/text-amber-800; added
dark:bg-amber-900/20 and lighter text for dark mode
