feat(media-hosting): add multi-provider configurable upload with fallback

Add Random/Preferred/None modes, preferred provider selection, and provider-order fallback. Catbox/Imgur/PostImages on Android via WebView; Electron CDP automation for Imgur/PostImages. Hide upload controls on web runtime.

src/lib/media-hosting/error-format.ts

@@ -0,0 +1,24 @@
+import type { ProviderId } from './types';
+
+export interface ProviderAttempt {
+  provider: ProviderId;
+  success: boolean;
+  error?: string;
+}
+
+export type TranslateFn = (key: string) => string;
+
+/**
+ * Formats aggregated error message when all providers fail (random mode).
+ */
+export function formatAggregatedError(attempts: ProviderAttempt[], t: TranslateFn): string {
+  const details = attempts.map((a) => `${a.provider}: ${a.error ?? 'unknown'}`).join('; ');
+  return `${t('upload_failed')}. ${t('upload_failed_all_providers')}: ${details}`;
+}
+
+/**
+ * Formats error for preferred mode with actionable guidance.
+ */
+export function formatPreferredModeError(errorMessage: string, t: TranslateFn): string {
+  return `${t('upload_failed')}: ${errorMessage}. ${t('upload_failed_preferred_guidance')}`;
+}