docs: add deprecation notice for macOS native app

[wilcorrea]

Context

The macOS native app (apps/macos/) has been discontinued and is no longer actively maintained. The project now focuses exclusively on the Tauri cross-platform app (apps/tauri/).

This needs to be clearly documented to avoid confusion for contributors and users.

Current State

• ❌ No indication that macOS app is deprecated
• ❌ Directory structure suggests both apps are actively maintained
• ❌ README doesn't clarify which app is the primary one

Proposed Changes

Choose one or more of these approaches:

Option 1: Rename Directory (Most Visible)

git mv apps/macos apps/macos-deprecated
# Update any references in CI/docs

Option 2: Add DEPRECATED.md

Create apps/macos/DEPRECATED.md:

# ⚠️ DEPRECATED

This macOS native app (Swift + AppKit + WebKit) has been **discontinued** as of February 2025.

## Why?

The project now focuses exclusively on the **Tauri cross-platform app** (`apps/tauri/`), which provides:
- Cross-platform support (macOS, Linux, Windows)
- Unified codebase and maintenance
- Better plugin ecosystem
- All features from the native app + new capabilities

## Migration

If you're using the macOS app, please switch to the Tauri version:
```bash
cd apps/tauri
npm install
npx tauri build

Archive

This code is kept for historical reference and learning purposes. It will not receive updates or bug fixes.

Last maintained: v0.2.0 (February 2025)

### Option 3: Update Main README
Add a clear section in the root README:
```markdown
## Applications

### Tauri (Primary) ✅
Cross-platform desktop app supporting macOS, Linux, and Windows.
- **Location**: `apps/tauri/`
- **Status**: Active development
- **Docs**: [Tauri Documentation](apps/tauri/README.md)

### macOS Native (Deprecated) ⚠️
Legacy macOS-only app built with Swift + AppKit.
- **Location**: `apps/macos/`
- **Status**: **Discontinued** (as of February 2025)
- **Replacement**: Use Tauri app instead

Option 4: Add Visual Indicator

Create apps/macos/README.md:

# ⚠️ DEPRECATED - macOS Native App

> **This app has been discontinued. Use [apps/tauri](../tauri/) instead.**

This directory contains the legacy macOS-only implementation and is kept for historical purposes only.

Recommended Approach

Implement Options 2 + 3 + 4:

1. Add DEPRECATED.md with full explanation
2. Add deprecation notice to macOS app's README
3. Update main README to clarify app status

OR go bold with Option 1 (rename directory) if you want maximum clarity.

Additional Cleanup (Optional)

Consider:

• Remove macOS app from CI workflows (if present)
• Archive macOS releases on GitHub
• Update website to only mention Tauri app
• Add deprecation banner to macOS app documentation

References

• CodeRabbit comment: feat: add offline voice-to-text with Whisper and UI improvements #2 (comment)
• PR discussion: feat: add offline voice-to-text with Whisper and UI improvements #2 (comment)

Priority

Medium - Prevents contributor confusion and wasted effort on deprecated code.