add_account: always append at end of file

The original "find last Open directive, insert after its metadata" logic
was a clever optimisation for the monolithic ledger where opens, txns,
and assertions all lived in one file -- you wanted new opens grouped with
existing opens, not appended after a long transaction tail.

Post-split, each include file has one mutation profile:
  - accounts/chart.beancount: only Open directives
  - accounts/users.beancount: only Open directives
  - transactions.beancount:   only Transactions

There is no longer a content shape that benefits from mid-file insertion;
the existing heuristic also had a pre-existing bug where it only matched
'open ' OR '{current_year}-' as line prefixes, so 1970-* seed opens were
invisible and the search "stuck" to the first current-year line in the
file (which on aio-demo ended up being the wrong place).

Drop the search; always append. Simpler, chronological, append-only
friendly.

Refs: aiolabs/libra#28

fava_client.py

@@ -1626,20 +1626,14 @@ class FavaClient:
                             logger.info(f"Account {account_name} already exists in {target_file}")
                             return {"data": sha256sum, "mtime": source_data.get("mtime", "")}
 
-                        # Step 3: Find insertion point (after last Open directive AND its metadata).
-                        # If the file has no Open directives yet (e.g. the empty
-                        # accounts/users.beancount seed), append at end of file
-                        # so the seed header comments stay at the top.
+                        # Step 3: Always append at end of file.
+                        # Post-split layout, each include file has one mutation
+                        # profile (only Open directives in chart/users, only
+                        # Transactions in transactions.beancount), so there's no
+                        # reason to slot new entries mid-file. Append-only also
+                        # keeps the seed header comments at the top and makes
+                        # the file's evolution trivially readable.
                         lines = source.split('\n')
-                        insert_index = None
-                        for i, line in enumerate(lines):
-                            if line.strip().startswith(('open ', f'{opening_date.year}-')) and 'open' in line:
-                                # Found an Open directive, now skip over any metadata lines
-                                insert_index = i + 1
-                                # Skip metadata lines (lines starting with whitespace)
-                                while insert_index < len(lines) and lines[insert_index].startswith((' ', '\t')) and lines[insert_index].strip():
-                                    insert_index += 1
-                        if insert_index is None:
                         insert_index = len(lines)
 
                         # Step 4: Format Open directive as Beancount text