Fix French accents, add validation, error handling, and browser compatibility

[Copilot]

Addresses all actionable review feedback from PR #57 focusing on i18n quality, input validation, error handling, and browser compatibility.

French Localization

• Fixed missing accents throughout: Français, Cybersécurité, Générale, préférée, Fréquence, Réinitialiser, etc.
• Updated config.json, issue templates, email templates, and UI strings

Input Validation & Security

Aggregator (src/aggregator.js)

• Config loading with actionable error messages on missing/malformed files
• Email validation (RFC-compliant regex) before sending
• API key format validation with sanitized error output
• Deduplication threshold type checking
• HTML lang attribute validation (ISO 639-1 codes only)

Workflows

• Email validation with abuse detection (max @ symbol threshold)
• RSS URL format validation
• Input sanitization for source names and tags (XSS prevention)

// Before: silent crash on missing config
const config = JSON.parse(fs.readFileSync(CONFIG_PATH, 'utf-8'));

// After: actionable error
try {
  if (!fs.existsSync(CONFIG_PATH)) {
    console.error(`ERROR: Configuration file not found at ${CONFIG_PATH}`);
    console.error('Please ensure config.json exists in the repository root.');
    process.exit(1);
  }
  // ...
} catch (e) {
  console.error(`ERROR: Failed to load or parse config.json: ${e.message}`);
  process.exit(1);
}

Browser Compatibility

• localStorage availability checks before all operations
• Try-catch wrappers with quota overflow recovery
• IntersectionObserver feature detection with graceful degradation
• ReadHistory.markRead safety guard against undefined

Rate Limiting & Configuration

• Email sending rate limit (100ms delay between sends)
• Configurable sender via EMAIL_FROM env var with dev domain warning
• Constants extracted for magic numbers (VALID_LANG_CODES, MAX_SOURCE_NAME_LENGTH, etc.)

Security Scan

✅ CodeQL: 0 alerts

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