v3.2.6-sr94.47: System, light and dark appearance

Accessible appearance modes

• Add System, Light and Dark preferences with early initialization, native keyboard controls, cross-tab synchronization and graceful handling of unavailable browser storage.
• Adjust management surfaces, chart labels, grid lines, tooltips, semantic colors and icons. Preserve white QR quiet zones and print sheets.
• Keep the existing light palette, custom-style precedence, APIs, authorization and public redirects unchanged. No migration or new dependency.
• Resolve static assets relative to the application directory so isolated or custom working-directory launches remain functional.

Validation: full source regression; rendered desktop/mobile workflows at 1440/390/320px across 15 routes in both themes; contrast, storage denial, keyboard, system changes, chart pixels and QR/print checks. Screenshot review prompted additional select-arrow and legacy-icon fixes.

Documentation: docs/THEMES.md. Release publication is separate from recoverable homelab deployment; the community ledger records deployment only after its live gates pass. Native Safari/Firefox and physical assistive-technology acceptance are not claimed by Chromium tests.