mobile: add a theme setting: light, dark, or follow the system

[xombiemp]

Summary

• Adds a plugin.theme option ('light', 'dark' or 'system') plus a live theme selector on the settings page, working like the existing accent selector: not persisted, a reload reverts to the configured option. 'system' follows the device's light/dark preference live via prefers-color-scheme.
• Everything themes off Bootstrap's data-bs-theme: the core's Bootstrap 5 assets flip their own components, and the hardcoded light colors in mobile.css move to the corresponding Bootstrap CSS variables.
• The near-black accent would be invisible on the dark theme's near-black background, so there it resolves to its counterpart, near-white — it was always really "maximum-contrast neutral", and its dropdown label becomes "Contrast" to match. On the near-white fills, the progress-bar stripes, progress labels and selected-filter text flip dark to stay legible.
• Progress-bar labels also get a bolder weight (500 → 700) in all themes for readability over the bar.

Test plan

• Settings → Theme: switch between Light / Dark / System and confirm every page (list, details, add, filter, settings, directory picker) follows
• With theme 'system', flip the device's dark mode and confirm the UI follows without a reload
• Dark theme + Contrast accent: buttons, tab highlights, selected filter rows, progress bars (stripes and labels) all stay legible
• Light theme unchanged with the default config

Made with Cursor