Modernize themes with integrated backgrounds

[bifrost0x]

What changed

• Reworked the color and surface treatment for all ten existing themes without changing layout or control positions.
• Added local decorative background assets and carried their visual language into the workspace, header, menus, modals, cards, notepad, login, and registration surfaces.
• Grouped the theme selector into Professional and Fun themes.
• Refined Paper Ops into a darker blue-gray Blueprint Steel direction with stronger color depth and readable contrast.
• Added validated local theme persistence so authentication pages inherit the last selected theme.
• Added focused browser coverage for typography stability, theme surfaces, backgrounds, authentication persistence, contrast, and Matrix semantic colors.

Why

Several themes were too visually extreme for sustained use, while background identity disappeared inside active workspaces and authentication screens. This update creates a stronger professional baseline while retaining intentionally playful themes such as Matrix, Retro, and Cyberpunk.

Impact

• No layout, position, backend, session, or persistence format changes.
• All background assets are bundled locally; no runtime CDN or external asset dependency is introduced.
• Existing theme identifiers remain unchanged.

Validation

• playwright test tests/e2e/theme-system.spec.js: 6 passed
• npm run lint:js: passed
• npm run test:js: 198 passed
• Focused Python template tests: 35 passed
• npm run vendor:check: passed
• git diff --check: passed