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/proofreading/Proofreader.ts

@@ -1,6 +1,6 @@
 import { Editor, Notice } from "obsidian";
 import { AIService } from "../AIService";
-import { AIProvider, PROVIDER_URLS } from "../SettingsManager";
+import { resolveChatProvider } from "../ProviderResolver";
 import NibbleAI from "main";
 
 export class Proofreader {
@@ -10,33 +10,6 @@ export class Proofreader {
 		this.plugin = plugin;
 	}
 
-	private resolveProvider(): { apiKey: string; endpoint: string; provider: AIProvider } | null {
-		const provider = this.plugin.settings.proofreadingProvider || "openrouter";
-		let apiKey = "";
-		let endpoint = PROVIDER_URLS[provider] || "";
-
-		switch (provider) {
-			case "openai":
-				apiKey = this.plugin.settings.openAiApiKey;
-				break;
-			case "anthropic":
-				apiKey = this.plugin.settings.anthropicApiKey;
-				break;
-			case "openrouter":
-				apiKey = this.plugin.settings.openRouterApiKey;
-				break;
-			case "custom":
-				apiKey = this.plugin.settings.customApiKey;
-				endpoint = this.plugin.settings.postProcessingUrl || "";
-				break;
-		}
-
-		if (!apiKey) return null;
-		if (!endpoint) return null;
-
-		return { apiKey, endpoint, provider };
-	}
-
 	async fixSelection(editor: Editor): Promise<void> {
 		const selection = editor.getSelection();
 		if (!selection || selection.trim().length === 0) {
@@ -44,7 +17,10 @@ export class Proofreader {
 			return;
 		}
 
-		const config = this.resolveProvider();
+		const config = resolveChatProvider(
+			this.plugin.settings,
+			this.plugin.settings.proofreadingProvider || "openrouter"
+		);
 		if (!config) {
 			new Notice(
 				"⚠️ No API key for selected provider. Check Settings \u2192 NibbleAI \u2192 API Keys."