feat: implement web UI for provider and model configuration

- Added 'provider_configs' and 'model_configs' tables to database.
- Refactored ProviderManager to support thread-safe dynamic updates and database overrides.
- Implemented 'Models' tab in dashboard to manage model visibility, mapping, and pricing.
- Added provider configuration modal to 'Providers' tab.
- Integrated database overrides into chat completion logic (enabled state, mapping, and cost).

static/js/pages/providers.js

@@ -124,7 +124,64 @@ class ProvidersPage {
     }
 
     configureProvider(id) {
-        window.authManager.showToast('Provider configuration via UI not yet implemented', 'info');
+        const provider = this.providers.find(p => p.id === id);
+        if (!provider) return;
+
+        const modal = document.createElement('div');
+        modal.className = 'modal active';
+        modal.innerHTML = `
+            <div class="modal-content">
+                <div class="modal-header">
+                    <h3 class="modal-title">Configure ${provider.name}</h3>
+                    <button class="modal-close" onclick="this.closest('.modal').remove()">
+                        <i class="fas fa-times"></i>
+                    </button>
+                </div>
+                <div class="modal-body">
+                    <div class="form-control">
+                        <label class="checkbox-label" style="display: flex; align-items: center; gap: 0.5rem; cursor: pointer;">
+                            <input type="checkbox" id="provider-enabled" ${provider.enabled ? 'checked' : ''} style="width: auto;">
+                            <span>Enable Provider</span>
+                        </label>
+                    </div>
+                    <div class="form-control">
+                        <label for="provider-base-url">Base URL</label>
+                        <input type="text" id="provider-base-url" value="${provider.base_url || ''}" placeholder="Default: ${provider.id === 'ollama' ? 'http://localhost:11434/v1' : 'Standard API URL'}">
+                    </div>
+                    <div class="form-control">
+                        <label for="provider-api-key">API Key (Optional / Overwrite)</label>
+                        <input type="password" id="provider-api-key" placeholder="••••••••••••••••">
+                        <small>Leave blank to keep existing key from .env or config.toml</small>
+                    </div>
+                </div>
+                <div class="modal-footer">
+                    <button class="btn btn-secondary" onclick="this.closest('.modal').remove()">Cancel</button>
+                    <button class="btn btn-primary" id="save-provider-config">Save Configuration</button>
+                </div>
+            </div>
+        `;
+        
+        document.body.appendChild(modal);
+        
+        modal.querySelector('#save-provider-config').onclick = async () => {
+            const enabled = modal.querySelector('#provider-enabled').checked;
+            const baseUrl = modal.querySelector('#provider-base-url').value;
+            const apiKey = modal.querySelector('#provider-api-key').value;
+            
+            try {
+                await window.api.put(`/providers/${id}`, {
+                    enabled,
+                    base_url: baseUrl || null,
+                    api_key: apiKey || null
+                });
+                
+                window.authManager.showToast(`${provider.name} configuration saved`, 'success');
+                modal.remove();
+                this.loadProviders();
+            } catch (error) {
+                window.authManager.showToast(error.message, 'error');
+            }
+        };
     }
 
     setupEventListeners() {