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.

README.md

@@ -75,11 +75,15 @@ Open **Settings** → **NibbleAI** and add at least one API key:
 
 | Key | Used For |
 |-----|----------|
-| **Transcription API Key** | Whisper transcription (OpenAI, Groq, Azure) |
-| **OpenAI API Key** | Proofreading, title generation, property filling, post-processing *(recommended: OpenRouter)* |
-| **Anthropic API Key** | Post-processing with Claude models |
+| **Groq API Key** | Whisper transcription, and as a chat provider for proofreading/title/property/post-processing |
+| **OpenAI API Key** | Whisper transcription, and as a chat provider for proofreading/title/property/post-processing |
+| **Anthropic API Key** | Claude models for proofreading/title/property/post-processing |
+| **OpenRouter API Key** | Access to hundreds of models for proofreading/title/property/post-processing |
+| **Custom API Key + URL** | Any OpenAI-compatible endpoint (self-hosted, Azure, etc.) |
 
-> **Tip:** For proofreading, title generation, and property filling, an [OpenRouter](https://openrouter.ai/keys) API key with a free model like `google/gemini-2.5-flash-lite` costs nothing and works great.
+Every LLM-backed feature (Proofreading, Title Generation, Property Filler, Post-Processing) shows the same provider dropdown — pick any provider that has a key configured above.
+
+> **Tip:** An [OpenRouter](https://openrouter.ai/keys) API key with a free model like `google/gemini-2.5-flash-lite` costs nothing and works great across all four AI features.
 
 ### 2. Try the Features
 
@@ -172,13 +176,14 @@ Securely stored in your system keychain via Obsidian's SecretStorage. Never writ
 | Setting | Default | Description |
 |---------|---------|-------------|
 | Enable | Off | Clean up transcripts with an LLM |
-| Provider | Anthropic | Anthropic, OpenAI, or Custom |
+| Provider | Anthropic | Any configured provider: Anthropic, OpenAI, Groq, OpenRouter, or Custom |
 | Model | `claude-sonnet-4-20250514` | LLM for cleanup |
 | Auto-generate title | Off | Create filenames from content |
 
 ### Proofreading
 | Setting | Default | Description |
 |---------|---------|-------------|
+| Provider | OpenRouter | Any configured provider: Anthropic, OpenAI, Groq, OpenRouter, or Custom |
 | Model | `openai/gpt-4o-mini` | Model for proofreading |
 | Temperature | `0.2` | Lower = more consistent |
 | Max tokens | `2000` | Maximum response length |
@@ -187,12 +192,16 @@ Securely stored in your system keychain via Obsidian's SecretStorage. Never writ
 ### Title Generation
 | Setting | Default | Description |
 |---------|---------|-------------|
+| Provider | OpenRouter | Any configured provider: Anthropic, OpenAI, Groq, OpenRouter, or Custom |
+| Model | `openai/gpt-4o-mini` | Model for generating titles |
 | Mode | Rename file | Rename file or set frontmatter property |
 | Max title length | 8 words | Maximum words in generated title |
 
 ### Property Filler
 | Setting | Default | Description |
 |---------|---------|-------------|
+| Provider | OpenRouter | Any configured provider: Anthropic, OpenAI, Groq, OpenRouter, or Custom |
+| Model | `openai/gpt-4o-mini` | Model for generating property values |
 | Custom prompt | *(built-in)* | Template with `{property}` placeholder |
 | Overwrite existing | Off | Replace existing property values |
 
@@ -205,7 +214,8 @@ NibbleAI/
 ├── main.ts                          # Plugin entry point, commands, events
 ├── src/
 │   ├── SettingsManager.ts           # Settings + SecretStorage management
-│   ├── AIService.ts                 # Unified AI client (OpenAI + Anthropic)
+│   ├── AIService.ts                 # Unified AI client (OpenAI-compatible + Anthropic-native)
+│   ├── ProviderResolver.ts          # Shared provider → API key/endpoint resolution
 │   ├── ModelBrowser.ts              # OpenRouter model suggest modal
 │   ├── PostProcessor.ts             # LLM post-processing for transcripts
 │   ├── NibbleAISettingsTab.ts       # Settings UI