Fix get_entry_context to use /source_slice endpoint

The /context endpoint returns entry metadata but not the editable source.
The /source_slice endpoint returns the actual source text and sha256sum
needed for approving/rejecting entries.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

fava_client.py

@@ -951,18 +951,17 @@ class FavaClient:
 
     async def get_entry_context(self, entry_hash: str) -> Dict[str, Any]:
         """
-        Get entry context including source text and sha256sum.
+        Get entry source text and sha256sum for editing.
+
+        Uses /source_slice endpoint which returns the editable source.
 
         Args:
             entry_hash: Entry hash from get_journal_entries()
 
         Returns:
             {
-                "entry": {...},  # Serialized entry
                 "slice": "2025-01-15 ! \"Description\"...",  # Beancount source text
                 "sha256sum": "abc123...",  # For concurrency control
-                "balances_before": {...},
-                "balances_after": {...}
             }
 
         Example:
@@ -973,7 +972,7 @@ class FavaClient:
         try:
             async with httpx.AsyncClient(timeout=self.timeout) as client:
                 response = await client.get(
-                    f"{self.base_url}/context",
+                    f"{self.base_url}/source_slice",
                     params={"entry_hash": entry_hash}
                 )
                 response.raise_for_status()