feat(accounts): validate account-name characters server-side

The endpoint only checked the root prefix, so a direct API call (bypassing
the UI) could write a malformed Open directive into the ledger source.
Add _validate_account_name mirroring Beancount's core/account.py grammar
(root [\p{Lu}][\p{L}\p{Nd}-]*, sub [\p{Lu}\p{Nd}][\p{L}\p{Nd}-]*, >=1
sub-account) — verified to match beancount.core.account.is_valid across
20 cases incl. Unicode, digit-start subs, hyphens. Align the client
segment regex to the same rule (was ASCII-only, rejected valid names).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

views_api.py

@@ -3661,6 +3661,52 @@ async def api_get_account_hierarchy(
 _VALID_ACCOUNT_PREFIXES = ("Assets:", "Liabilities:", "Equity:", "Income:", "Expenses:")
 
 
+def _is_valid_account_component(component: str, *, is_root: bool) -> bool:
+    """Validate one ':'-separated account component against Beancount's grammar.
+
+    Mirrors core/account.py: a root component matches ``[\\p{Lu}][\\p{L}\\p{Nd}-]*``
+    (must start with an uppercase letter); a sub component matches
+    ``[\\p{Lu}\\p{Nd}][\\p{L}\\p{Nd}-]*`` (may also start with a digit). Body
+    chars are letters, decimal digits, or hyphen. Implemented with Unicode-aware
+    str methods (libra's runtime has no beancount — Fava is a separate service),
+    so non-ASCII letters are accepted exactly as Beancount accepts them.
+    """
+    if not component:
+        return False
+    first, rest = component[0], component[1:]
+    first_ok = (first.isalpha() and first.isupper()) or (
+        not is_root and first.isdecimal()
+    )
+    if not first_ok:
+        return False
+    return all(ch == "-" or ch.isalpha() or ch.isdecimal() for ch in rest)
+
+
+def _validate_account_name(name: str) -> None:
+    """Raise HTTP 400 if ``name`` is not a syntactically valid Beancount account.
+
+    The UI guards this client-side, but the endpoint is reachable directly via
+    API, so this is the load-bearing check before the name is written into the
+    ledger source. Requires a root plus at least one sub-component.
+    """
+    parts = name.split(":")
+    valid = (
+        len(parts) >= 2
+        and _is_valid_account_component(parts[0], is_root=True)
+        and all(_is_valid_account_component(p, is_root=False) for p in parts[1:])
+    )
+    if not valid:
+        raise HTTPException(
+            status_code=HTTPStatus.BAD_REQUEST,
+            detail=(
+                f"Invalid account name {name!r}: each ':'-separated part must be "
+                "letters/digits/hyphens, the root starting with an uppercase "
+                "letter (sub-accounts may start with a digit), with at least one "
+                "sub-account (e.g. Expenses:Food)."
+            ),
+        )
+
+
 @libra_api_router.post("/api/v1/admin/accounts", status_code=HTTPStatus.CREATED)
 async def api_admin_add_chart_account(
     payload: CreateChartAccount,
@@ -3685,6 +3731,8 @@ async def api_admin_add_chart_account(
             ),
         )
 
+    _validate_account_name(payload.name)
+
     logger.info(
         f"Admin {auth.user_id[:8]} adding chart account {payload.name} "
         f"with currencies {payload.currencies}"