Improves Beancount entry generation and sanitization

Adds a function to sanitize strings for use as Beancount links,
ensuring compatibility with Beancount's link restrictions.

Refactors the journal entry creation process to use EUR-based
postings when fiat currency is provided, improving accuracy
and consistency. The legacy SATS-based fallback is retained for
cases without fiat currency information.

Adjusts reference generation for Beancount entries using the
sanitized description.

helper/import_beancount.py

@@ -238,6 +238,30 @@ class AccountLookup:
 
 # ===== CONVERSION FUNCTIONS =====
 
+def sanitize_link(text: str) -> str:
+    """
+    Sanitize a string to make it valid for Beancount links.
+
+    Beancount links can only contain: A-Z, a-z, 0-9, -, _, /, .
+    All other characters are replaced with hyphens.
+
+    Examples:
+        >>> sanitize_link("Test (pending)")
+        'Test-pending'
+        >>> sanitize_link("Invoice #123")
+        'Invoice-123'
+        >>> sanitize_link("import-20250623-Action Ressourcerie")
+        'import-20250623-Action-Ressourcerie'
+    """
+    import re
+    # Replace any character that's not alphanumeric, dash, underscore, slash, or period with a hyphen
+    sanitized = re.sub(r'[^A-Za-z0-9\-_/.]', '-', text)
+    # Remove consecutive hyphens
+    sanitized = re.sub(r'-+', '-', sanitized)
+    # Remove leading/trailing hyphens
+    sanitized = sanitized.strip('-')
+    return sanitized
+
 def eur_to_sats(eur_amount: Decimal, btc_eur_rate: float) -> int:
     """Convert EUR to satoshis using BTC/EUR rate"""
     btc_amount = eur_amount / Decimal(str(btc_eur_rate))
@@ -454,10 +478,13 @@ def convert_to_castle_entry(parsed: dict, btc_eur_rate: float, account_lookup: A
             "metadata": metadata
         })
 
+    # Create sanitized reference link
+    desc_part = sanitize_link(parsed['description'][:30])
+
     return {
         "description": parsed['description'],
         "entry_date": parsed['date'].isoformat(),
-        "reference": f"import-{parsed['date'].strftime('%Y%m%d')}-{parsed['description'][:20].replace(' ', '-')}",
+        "reference": f"import-{parsed['date'].strftime('%Y%m%d')}-{desc_part}",
         "flag": "*",
         "meta": {
             "source": "beancount_import",