feat(plugins): add plugin settings modal and registration contract

[JamesDAdams]

Summary

Adds plugin settings modal and registration capability to OpenFox. Plugins can now register custom configuration options (via dynamic form fields or a hosted custom UI iframe) using registry.registerSettings() or by specifying "hasSettings": true in package.json under openfox.

• Backend (src/server/routes/plugins.ts, src/server/providers/plugins/):
  • Extended ProviderPluginRegistry and ProviderRegistry with registerSettings and registerSettingsForPlugin.
  • Added GET /api/plugins/:name/settings and POST /api/plugins/:name/settings endpoints for reading and saving plugin configuration.
  • Implemented password field sanitization (secrets are never echoed in GET responses and are merged server-side when updated).
  • Server-side required-field validation.
• Frontend (web/src/components/settings/):
  • Created PluginSettingsModal supporting dynamic input types (text, password, number, boolean, select, textarea) and custom UI URL embeds.
  • Added a conditional Settings button to installed plugin cards in PluginsTab (only displayed when settings are available).
  • Client-side required-field validation before submitting.
• Documentation:
  • Updated README.md with examples for registering plugin settings and declaring openfox.hasSettings.

AI-Enhanced Development

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• AI Models: Gemini 3.6 Flash / hy3

Cache Impact

Does this PR affect anything cached — system prompts, tool definitions, skills, or other context?

• No

[JamesDAdams]

@co-l I make the change for i18n translate