Refactor liquid glass surfaces and dark mode styling

From the PR description

Summary

• Consolidates liquid-glass styling into shared flat, subtle, float, modal, and translucent surface tiers.
• Adds unified hover, selected, pressed, and modal-row interaction states with light and dark theme values.
• Moves dark mode into a dedicated Settings > Appearance page and expands semantic dark-mode coverage across app surfaces.

What changed

• Centralized surface backgrounds, borders, shadows, and interaction colors in shared web and Word add-in tokens.
• Migrated tables, sidebars, side panels, inputs, dropdowns, pills, modal rows, chat overlays, auth screens, and workflow surfaces to the shared classes.
• Preserved the original tab-pill selected treatment while standardizing other selected and hover states.
• Added selected backgrounds to shared table-filter dropdowns and reduced Model Toggle rounding.
• Added dark-mode-aware document, editor, citation, highlight, spreadsheet, and chat fade colors.
• Consolidated MikeIcon into one shared implementation with thin web and Word adapters.
• Replaced the citation verification information menu with an accessible popover.
• Removed obsolete CFR/USC styling, unused color tokens, hard-coded focus fills, and the APP_SURFACE compatibility aliases.
• Updated design-system documentation and regression coverage.

Why

Surface styling had diverged across features and many components carried copied light-only backgrounds, borders, and shadow recipes. Centralizing the material tiers and interaction states gives the app consistent depth, makes dark mode apply without component-by-component shadow overrides, and reduces future styling drift between the web app and Word add-in.

Testing performed

• Targeted frontend Vitest suites for liquid surfaces, table filters, dropdowns, Model Toggle, citation verification, MikeIcon, settings, and modal interactions.
• npx tsc --noEmit --project frontend/tsconfig.json
• npm run build --prefix frontend
• npm run typecheck --prefix word-addin
• npm run build:e2e --prefix word-addin
• npm run lint --prefix frontend (no errors; existing warnings remain)
• git diff --check