Fix Fava API endpoint for getting entry context

Use GET /api/context instead of GET /api/source_slice. Fava's API
naming convention means source_slice only supports PUT and DELETE,
while context is the correct endpoint for reading entry data.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

fava_client.py

@@ -10,7 +10,9 @@ Fava provides a REST API for:
 - Querying balances (GET /api/query)
 - Balance sheets (GET /api/balance_sheet)
 - Account reports (GET /api/account_report)
-- Updating/deleting entries (PUT/DELETE /api/source_slice)
+- Getting entry context (GET /api/context)
+- Updating entries (PUT /api/source_slice)
+- Deleting entries (DELETE /api/source_slice)
 
 See: https://github.com/beancount/fava/blob/main/src/fava/json_api.py
 """
@@ -1098,7 +1100,8 @@ class FavaClient:
         """
         Get entry source text and sha256sum for editing.
 
-        Uses /source_slice endpoint which returns the editable source.
+        Uses /context endpoint which returns the editable source slice.
+        Note: Fava's API uses get_context for reading, put_source_slice for writing.
 
         Args:
             entry_hash: Entry hash from get_journal_entries()
@@ -1117,7 +1120,7 @@ class FavaClient:
         try:
             async with httpx.AsyncClient(timeout=self.timeout) as client:
                 response = await client.get(
-                    f"{self.base_url}/source_slice",
+                    f"{self.base_url}/context",
                     params={"entry_hash": entry_hash}
                 )
                 response.raise_for_status()