[OpenSpec] sync-theming-on-token-change

[rubenvdlinde]

Summary

When an admin selects a different token set in nldesign, Nextcloud's built-in theming values (primary color, background color, logo, background image) remain unchanged. This change adds:

1. Theming sync dialog — After token set selection, show a confirmation dialog comparing current vs. proposed Nextcloud theming values and offer to update automatically
2. Dropdown token selector — Replace radio buttons with a searchable <select> dropdown to handle 400+ token sets

Tasks

• 1.1 Add theming metadata to token-sets.json entries
• 1.2 Create img/logos/ and img/backgrounds/ directories with initial images
• 2.1 Update TokenSetService to include theming metadata in output
• 3.1 Add GET /settings/theming endpoint
• 3.2 Add POST /settings/theming endpoint for colors
• 3.3 Add image upload support to POST /settings/theming
• 4.1 Replace radio buttons with select dropdown in admin template
• 4.2 Update admin.js for dropdown change handler
• 5.1 Add dialog HTML and CSS
• 5.2 Add dialog trigger logic after token set change
• 5.3 Wire up dialog confirm/cancel actions
• 6.1 End-to-end test: select token set with theming, confirm dialog, verify NC theming updated
• 6.2 Test dropdown with all token sets, verify alphabetical sorting and selection