fix: dark/light contrast on 6 chrome surfaces

[ParasxAgarwal]

What

Six small, objective dark/light contrast fixes in the app chrome. className-only — no behavior, store, or layout change.

Dark mode was missing dark: twins on a few surfaces, and two elements had the inverse bug (hardcoded white text that is unreadable in light mode):

Missing dark twins (look wrong in dark mode):

• DropDown menu panel — dividers + outline ring had no dark variant → add dark:divide-slate-700 + dark:ring-white/10 (matches the menu flyout outline).
• ViewAllBookmarks panel — the bare border defaulted to a light gray with no dark color → add border-slate-300 + dark:border-slate-700.
• Toggle switch knob — light-only knob border → add dark:after:border-gray-500.
• AboutDialog tagline — text-gray-500 too dim on the dark panel → add dark:text-gray-400.

Inverse bugs (unreadable in light mode):

• ConfirmDialog title was text-white on the modal panel, which is bg-slate-200 in light mode → white-on-light-gray, invisible. Now text-slate-800 dark:text-white.
• Notification text was text-white inside the menu flyout, which is bg-slate-100 in light mode → white-on-light, invisible. Now inherits in light, dark:text-white.

Intentionally NOT changed

The three bg-white surfaces in Previewer/Device (the device viewport behind the <webview>) and DesignOverlay (the design-mockup backdrop) are the canvas, not chrome: the loaded website / uploaded design paints over them, so they must stay neutral in every theme. Darkening them would flash dark behind transparent or short pages. Left as-is by design.

Verification (project toolchain)

• prettier --check: all matched
• eslint on the 6 files: clean (exit 0)
• tsc --noEmit: clean (exit 0, 0 errors)
• vitest run (full suite): 14 files / 69 tests passed

Note on the commit

Committed with --no-verify. The repo's husky pre-commit hook fails to spawn cross-env in this monorepo checkout (the binary lives under desktop-app/node_modules/.bin while the hook runs from the repo root → ENOENT before it does any work). The checks the hook performs (prettier + eslint via lint-staged) were run and pass independently, as shown above, and CI lints via the lint script rather than the hook.