Info and Warning Banners for Welcome Screen

[keyboardstaff]

Adds a modular, extensible banner system to the Welcome Screen that displays contextual info/warning/error messages.

Files Changed:

• python/api/banners.py (new) - Backend API endpoint for additional banners
• webui/js/banner-checks.js (new) - Frontend banner check functions
• webui/components/welcome/welcome-store.js (modified) - Banner state management and API calls
• webui/components/welcome/welcome-screen.html (modified) - Banner UI rendering

Why banner-checks.js is in webui/js/ instead of webui/components/welcome/:

• It contains pure utility functions (no Alpine.js store)
• Follows project convention: webui/js/ for shared utilities (like api.js, messages.js), webui/components/*/ for component-specific stores
• May be reused by other components in the future, as banner checks are expected to expand and evolve over time

Architecture:

• Frontend runs checks first, then calls backend /banners API for additional banners
• Banners are merged, deduplicated by ID, and sorted by priority
• Easy to extend: add functions to bannerChecks array (frontend) or checks list (backend)

Implemented Checks:

1. ✅ Unsecured connection warning (non-local access without credentials)
2. ✅ Credentials unencrypted warning (non-HTTPS with credentials)
3. ✅ Missing API key error (for all model providers, skips Ollama/LM Studio, skips HuggingFace for Embedding only)

Banner Structure:

{ id, type, priority, title, html, dismissible, source }