Fix theme toggle reliability and improve light/dark readability

[podcctv]

Motivation

• Users reported that the UI could not reliably switch between light and dark modes and that muted text had poor readability in both themes.
• The goal is to make theme switching robust across pages and improve visual contrast for critical UI text and cards.

Description

• Rewrote the theme wiring in master-api/static/theme.js to bind to any existing .theme-toggle elements and avoid injecting duplicate controls, and to provide a floating fallback toggle when no in-layout toggle exists.
• Improved theme state handling by validating stored values (localStorage) and falling back to the system preference via prefers-color-scheme, and synchronized color-scheme via html.style.colorScheme when applying a theme.
• Unified toggle UI updates so both new markup (.theme-icon) and legacy markup (.theme-toggle-icon) are refreshed and accessibility aria-label/title are kept correct after changes.
• Enhanced master-api/static/theme.css with higher-contrast --text-muted/--text-dimmed values, added floating toggle styles, legacy toggle compatibility styles, and targeted light-theme adjustments for headers, cards and sidebar items to improve overall readability.

Testing

• Ran a JavaScript syntax check with node --check master-api/static/theme.js, which completed successfully.
• Verified the changes are limited to the two files master-api/static/theme.js and master-api/static/theme.css via local diff inspection.
• No automated browser/visual tests were available in this environment to show render screenshots, so visual verification should be performed in a browser after deployment.

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Codex Task