test: pr1441 IME helper guards + pr1439 ja locale parity

- Loosen test_ime_composition._ime_guarded_enter_pattern to accept the
  new _isImeEnter(e) helper (PR #1441 widened guard for Safari + 229 keyCode
  + manual _imeComposing flag). Original e.isComposing-only pattern still
  matches via alternation.
- Add test_pr1441_ime_safari_guard.py (6 tests): pin the 3-guard helper,
  compositionstart sets manual flag, compositionend defers reset to next
  tick (Safari race), null-guard $('msg') for non-chat pages, send-Enter
  uses helper, dropdown-Enter uses helper.
- Add test_japanese_locale.py (8 tests): mirror Chinese/Korean templates,
  block exists, representative translations, full key parity with English,
  no extra keys, duplicates mirror en exactly, placeholders preserved,
  arrow-function values mirrored, _label uses Japanese script.

tests/test_pr1441_ime_safari_guard.py

@@ -0,0 +1,111 @@
+"""Regression tests for PR #1441 — IME composition Enter on Safari + broader IME coverage.
+
+Original guard was `e.isComposing` only, which fails on Safari where the committing
+keydown for IME composition fires AFTER `compositionend` with `isComposing=false`.
+PR #1441 adds two more guards (`keyCode===229` + manual `_imeComposing` flag).
+
+These tests pin the structural shape of the helper so a future cleanup pass that
+strips one of the three guards trips a test before shipping.
+"""
+
+import pathlib
+import re
+
+REPO_ROOT = pathlib.Path(__file__).parent.parent.resolve()
+BOOT_JS = (REPO_ROOT / "static" / "boot.js").read_text(encoding="utf-8")
+
+
+def test_ime_helper_function_exists():
+    """The `_isImeEnter` helper must exist and combine all 3 guards."""
+    # Helper definition — single function, three guards joined by ||
+    pattern = re.compile(
+        r"function\s+_isImeEnter\s*\(\s*e\s*\)\s*\{[^}]*"
+        r"e\.isComposing"
+        r"[^}]*"
+        r"e\.keyCode\s*===\s*229"
+        r"[^}]*"
+        r"_imeComposing"
+        r"[^}]*\}",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "_isImeEnter helper must combine e.isComposing, keyCode===229, and "
+        "_imeComposing flag (PR #1441)"
+    )
+
+
+def test_compositionstart_sets_manual_flag():
+    """A compositionstart listener on #msg must set _imeComposing = true."""
+    # The listener registers on the resolved `_c` (i.e. $('msg')) element
+    pattern = re.compile(
+        r"compositionstart['\"]\s*,\s*(?:\(\s*\)|function\s*\(\s*\))\s*=>?\s*\{?\s*"
+        r"_imeComposing\s*=\s*true",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "compositionstart listener must set _imeComposing = true (PR #1441)"
+    )
+
+
+def test_compositionend_resets_flag_on_next_tick():
+    """compositionend must reset _imeComposing in a setTimeout(..., 0) — NOT
+    synchronously — so Safari's trailing committing-Enter keydown is still
+    swallowed (it fires AFTER compositionend).
+    """
+    pattern = re.compile(
+        r"compositionend['\"]\s*,\s*(?:\(\s*\)|function\s*\(\s*\))\s*=>?\s*\{?\s*"
+        r"setTimeout\s*\(\s*(?:\(\s*\)|function\s*\(\s*\))\s*=>?\s*\{?\s*"
+        r"_imeComposing\s*=\s*false",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "compositionend listener must reset _imeComposing in setTimeout(..., 0) "
+        "to handle Safari's post-compositionend trailing Enter (PR #1441)"
+    )
+
+
+def test_ime_listeners_null_guard_msg_lookup():
+    """The IIFE that registers composition listeners must null-guard $('msg') so
+    boot.js does not throw on pages that don't have a #msg textarea (e.g. login,
+    onboarding).
+    """
+    # The IIFE pattern: (()=>{const _c=$('msg');if(!_c)return; ...
+    pattern = re.compile(
+        r"\(\s*\(\s*\)\s*=>\s*\{\s*const\s+_c\s*=\s*\$\(\s*['\"]msg['\"]\s*\)\s*;\s*"
+        r"if\s*\(\s*!\s*_c\s*\)\s*return\s*;",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "Composition-listener IIFE must null-guard $('msg') so non-chat pages "
+        "(login, onboarding) don't throw (PR #1441)"
+    )
+
+
+def test_chat_send_enter_uses_helper():
+    """The send-Enter path must call _isImeEnter(e), not e.isComposing."""
+    # The original was `if(e.isComposing){return;}` inside `if(e.key==='Enter')`.
+    # Now it must be `if(_isImeEnter(e)){return;}`.
+    pattern = re.compile(
+        r"if\s*\(\s*e\.key\s*===\s*['\"]Enter['\"]\s*\)\s*\{\s*"
+        r"if\s*\(\s*_isImeEnter\s*\(\s*e\s*\)\s*\)\s*",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "Chat composer send-Enter path must use _isImeEnter(e) helper (PR #1441)"
+    )
+
+
+def test_dropdown_enter_uses_helper():
+    """The autocomplete-dropdown Enter path must also use _isImeEnter(e).
+
+    Otherwise IME-confirming Enter inside a slash-command dropdown would
+    select the highlighted item instead of just committing the IME candidate.
+    """
+    pattern = re.compile(
+        r"if\s*\(\s*e\.key\s*===\s*['\"]Enter['\"]\s*&&\s*!\s*e\.shiftKey\s*\)\s*\{\s*"
+        r"if\s*\(\s*_isImeEnter\s*\(\s*e\s*\)\s*\)\s*",
+        re.DOTALL,
+    )
+    assert pattern.search(BOOT_JS), (
+        "Command-dropdown Enter path must use _isImeEnter(e) helper (PR #1441)"
+    )