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.

static/sw.js

@@ -7,7 +7,7 @@
 //   - Other static assets (images/fonts/libs): cache-first with bg refresh.
 //   - API / non-GET: never cached.
 // Bump CACHE_NAME whenever the precache list or SW logic changes.
-const CACHE_NAME = 'odysseus-v654-css-split-3';
+const CACHE_NAME = 'odysseus-v655-email-library-split';
 
 // KaTeX resolves these from its own stylesheet, so caching the CSS without them
 // gives offline math fallback glyphs instead of proper typesetting.
@@ -84,9 +84,12 @@ const PRECACHE = [
   '/static/js/slashCommands.js?v=20260921chatgptusage1',
   '/static/js/research/jobs.js?v=20260910researcherrorpersist1',
   '/static/js/emailInbox.js?v=20260914aireply4',
+  '/static/js/emailLibrary/index.js',
   '/static/js/emailLibrary/utils.js',
   '/static/js/emailLibrary/signatureFold.js',
   '/static/js/emailLibrary/state.js',
+  // emailInbox.js has always imported this one; it was never precached.
+  '/static/js/emailLibrary/replyRecipients.js',
   '/static/js/notes.js?v=20260911notesselectioncancel1',
   '/static/js/tasks.js?v=20260914taskmodel1',
   '/static/js/calendar.js?v=20260914emailsource11',