Fix white screen on display page by self-hosting Inter font

[dadachi]

Summary

• Display page (display/shops/show) would turn completely white when NormalDisplaySettingsController#update_text was called from the iOS client
• Root cause: rel="stylesheet" for Google Fonts is render-blocking. When the after_update_commit callback triggers a full page reload via Turbo Stream, the browser blocks rendering until the font CSS loads. If the request is slow or hangs, the page stays white
• Fix: self-host Inter font files with font-display: swap and serve via the asset pipeline, removing the external Google Fonts dependency entirely
• Removed Google Fonts domains from CSP since fonts are now self-hosted

Test plan

• Open display page for a shop in normal mode
• From iOS client, call update_text on NormalDisplaySettings
• Verify the display page reloads without turning white
• Verify Inter font still loads correctly after page render
• Test with slow network conditions (e.g., throttle in DevTools)

🤖 Generated with Claude Code