refactor(email): move emailLibrary.js into static/js/emailLibrary/

The email library was 11,375 lines in one file, the second-largest JS
module in the repo. `static/js/emailLibrary/` already held four extracted
helpers, so the package existed; the bulk of the code just was not in it.

The implementation moves to `emailLibrary/index.js` and the old path
becomes a re-export wrapper. Five call sites import that path, four of
them dynamically with a `?v=` string, and `sw.js` caches URLs verbatim,
so a wrapper is what makes the move need no coordinated edit to any of
them.

The test side is the part worth reviewing. 58 tests read
`static/js/emailLibrary.js` as text. Pointing them at
`emailLibrary/index.js` would buy one move and break again on the next
one, which is exactly what happened to the stylesheet tests (they now go
through `tests/helpers/stylesheets.py`). So the same shape:
`tests/helpers/js_modules.py` reads the whole package, and assertions
stop caring which module a function sits in.

`tests/test_email_library_module_graph_js.py` is new. This frontend has
no module-graph validation, and a package fails in ways a single file
cannot: a wrapper that drops an export is `undefined` at call time rather
than an error at load time, and a module that reads a `const` across an
import cycle throws only when that module is entered first. It pins the
wrapper's surface against the entry module's, evaluates every module on
its own in a browser, and requires both import paths to hand out one
instance.

It also caught a live gap while being written:
`emailLibrary/replyRecipients.js` is imported by `emailInbox.js`, an app
shell module, and was never in the `sw.js` precache.

tests/test_external_context_tool_gate.py

@@ -8,6 +8,7 @@ from pathlib import Path
 
 import pytest
 
+from tests.helpers.js_modules import email_library_paths
 import src.tool_capabilities as tool_capabilities
 from src.tool_capabilities import (
     KNOWN_CAPABILITY_TOOLS,
@@ -1405,11 +1406,10 @@ def test_frontend_tool_approval_uses_opaque_id_and_fixed_decisions():
             "static/js/chatStream.js",
             "static/js/document.js",
             "static/js/emailInbox.js",
-            "static/js/emailLibrary.js",
             "static/js/settings.js",
             "static/js/slashCommands.js",
         )
-    ]
+    ] + [p.read_text() for p in email_library_paths(include_wrapper=True)]
     assert all(
         "20260722emailfastindex1" not in source
         for source in approval_module_sources