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_security_regressions.py

@@ -18,6 +18,7 @@ import importlib
 from pathlib import Path
 
 import pytest
+from tests.helpers.js_modules import email_library_source
 
 
 # ── prompt-injection context wrapper ────────────────────────────
@@ -1009,8 +1010,7 @@ def test_diagnostics_routes_are_admin_gated():
 def test_email_thread_rendering_sanitizes_body_html():
     """Both threaded render paths must run server-parsed body_html through the
     allowlist sanitizer (the flat path already did)."""
-    src = Path(__file__).resolve().parents[1] / "static" / "js" / "emailLibrary.js"
-    text = src.read_text()
+    text = email_library_source()
     # every `t.body_html` reference is wrapped by _sanitizeHtml(...)
     assert text.count("t.body_html") == text.count("_sanitizeHtml(t.body_html")
     assert "t.body_html" in text  # guard against the file being refactored away