perf(ui): stop session loading from blocking shell (#5927)

* perf(ui): stop session loading from blocking shell

* fix(startup): open routes on their own data, retire the loader for good

Follow-up to review on #5927.

- Route openers are now classified by the data they actually read. Only
  /email touches the hydrated session list (its new-chat path falls back to
  the most recent session's model when no default chat is set), so every
  other route opens as soon as module wiring completes instead of queueing
  behind /api/sessions. This is the deferred-route half of #5926, which the
  first pass left unimplemented.
- index.html's 5s fallback removes the loader node again. Leaving it in the
  DOM indefinitely kept _shouldPreserveStartupComposer true forever on a
  hung /api/sessions, so the composer stopped clearing on session switch.
- A missing session module settles hydration instead of leaving the sidebar
  on "Loading chats…" and dropping the user's route on the floor.
- Startup sequencing moved to static/js/startupShell.js so it can be run by
  tests. The source-text assertions in test_startup_shell_session_loading.py
  are replaced by node-driven behavioural tests, per tests/TESTING_STANDARD.md.
- Reverted the unrequested loader a11y rework, removed the duplicated inert
  writes (the module stops the wave interval through a callback), and moved
  the bootstrap row's inline styles into .session-list-bootstrap.

* fix: preserve session bootstrap failure state

---------

Co-authored-by: Léo <leograndcontact@gmail.com>

static/style.css

@@ -38015,6 +38015,12 @@ button.cal-add-btn.cal-add-btn-text.cal-add-btn-sm:hover .cal-add-label {
   outline-offset: 2px;
   border-radius: 5px;
 }
+/* Bootstrap row shown while the session list hydrates, and on load failure.
+   Reads as a normal list row but is not selectable. */
+.session-list-bootstrap {
+  cursor: default;
+  pointer-events: none;
+}
 #email-lib-grid .date-section-header {
   padding: 10px 5px 3px;
 }