Hide light/system theme options until implemented

[0xMMA]

Context

Settings > General > Theme offers Dark, Light, and System options, but only Dark is properly styled.

Problem

Selecting "Light" removes the .app-dark class (shell.component.ts:159-160), which drops PrimeNG into its default Aura light mode — but no custom light theme CSS variables are defined in styles.scss. The result is an unstyled/broken-looking UI.

"System" is also non-functional — it's accepted as a value but treated as dark (settings.theme_preference !== 'light'), never actually reading OS preference.

Relevant Code

• frontend/src/app/features/settings/settings.component.ts:405-409 — theme dropdown options
• frontend/src/app/layout/shell.component.ts:157-161 — applyTheme() logic
• frontend/src/styles.scss:16-36 — only .app-dark variables defined, no light counterpart

Acceptance Criteria

• Light and System options removed from theme dropdown
• Theme preference locked to Dark until light theme is implemented