Unify AI provider selection across all features

Every LLM-backed feature (Post-Processing, Proofreading, Title
Generation, Property Filler) now reads from one CHAT_PROVIDERS
registry instead of 5 hand-duplicated provider lists, so they always
offer the exact same providers in the exact same order.

- Add Groq as a 5th chat-completion provider (was transcription-only
  despite its OpenAI-compatible API and an already-collected key).
- Provider dropdowns disable options with no API key configured
  (shown as e.g. "Anthropic (no API key)") instead of silently
  failing at runtime when picked.
- Collapse 3 byte-for-byte duplicated resolveProvider() methods
  (Proofreader, TitleGenerator, PropertyFiller) plus AudioHandler's
  getPostProcessingApiKey() into one resolveChatProvider() helper
  (src/ProviderResolver.ts).
- Fix Custom provider: previously Proofreading/Title/Property silently
  reused Post-Processing's postProcessingUrl with no visible field to
  set it. Renamed to a shared customApiUrl field with its own UI
  (createCustomProviderFields), now exposed on every tab that offers
  Custom, plus rows on the API Keys tab. Migrates old data.json values
  automatically.
- Update README settings reference and getting-started sections to
  match.

No behavior change for existing single-provider setups; migration
handles the postProcessingUrl -> customApiUrl rename transparently.

src/ProviderResolver.ts

@@ -0,0 +1,27 @@
+import { CHAT_PROVIDERS, PROVIDER_URLS, PluginSettings, PostProcessingProvider } from "./SettingsManager";
+
+export interface ResolvedChatProvider {
+	apiKey: string;
+	endpoint: string;
+	provider: PostProcessingProvider;
+}
+
+/**
+ * Resolves the API key + endpoint for a chat-completion provider selection.
+ * Shared by every AI feature that talks to an LLM (Post-Processing,
+ * Proofreading, Title Generation, Property Filler) so they all treat
+ * providers — including "no key configured" — identically.
+ */
+export function resolveChatProvider(
+	settings: PluginSettings,
+	provider: PostProcessingProvider
+): ResolvedChatProvider | null {
+	const info = CHAT_PROVIDERS.find((p) => p.id === provider);
+	if (!info) return null;
+
+	const apiKey = settings[info.apiKeyField];
+	const endpoint = provider === "custom" ? settings.customApiUrl : PROVIDER_URLS[provider];
+
+	if (!apiKey || !endpoint) return null;
+	return { apiKey, endpoint, provider };
+}