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_calendar_visual_static.py

@@ -2,6 +2,7 @@ from pathlib import Path
 import re
 
 from tests.helpers.stylesheets import app_css
+from tests.helpers.js_modules import email_library_source
 
 
 ROOT = Path(__file__).resolve().parents[1]
@@ -285,13 +286,10 @@ def test_calendar_tool_guidance_preserves_manual_tags_on_unrelated_updates():
 
 def test_calendar_visual_asset_versions_are_bumped():
     versions = []
-    for rel in (
-        "static/app.js",
-        "static/js/chatRenderer.js",
-        "static/js/emailInbox.js",
-        "static/js/emailLibrary.js",
-    ):
-        src = (ROOT / rel).read_text()
+    for src in [
+        (ROOT / rel).read_text()
+        for rel in ("static/app.js", "static/js/chatRenderer.js", "static/js/emailInbox.js")
+    ] + [email_library_source()]:
         match = re.search(r"calendar\.js\?v=([A-Za-z0-9_-]+)", src)
         assert match
         versions.append(match.group(1))