feat(electrobun): project wizard phases 1-5 (WIP)

- sanitize.ts: input sanitization (trim, control chars, path traversal)
- provider-scanner.ts: detect Claude/Codex/Ollama/Gemini availability
- model-fetcher.ts: live model lists from 4 provider APIs
- ModelConfigPanel.svelte: per-provider config (thinking, effort, sandbox, temperature)
- WizardStep1-3.svelte: split wizard into composable steps
- CustomDropdown/Checkbox/Radio: themed UI components
- provider-handlers.ts: provider.scan + provider.models RPC
- Wire providers into wizard step 3 (live detection + model lists)
- Replace native selects in 5 settings panels with CustomDropdown

ui-electrobun/src/bun/handlers/provider-handlers.ts

@@ -0,0 +1,30 @@
+/**
+ * Provider RPC handlers — scanning + model fetching.
+ */
+
+import { scanAllProviders } from '../provider-scanner.ts';
+import { fetchModelsForProvider } from '../model-fetcher.ts';
+
+export function createProviderHandlers() {
+  return {
+    'provider.scan': async () => {
+      try {
+        const providers = await scanAllProviders();
+        return { providers };
+      } catch (err) {
+        console.error('[provider.scan]', err);
+        return { providers: [] };
+      }
+    },
+
+    'provider.models': async ({ provider }: { provider: string }) => {
+      try {
+        const models = await fetchModelsForProvider(provider);
+        return { models };
+      } catch (err) {
+        console.error('[provider.models]', err);
+        return { models: [] };
+      }
+    },
+  };
+}