feat(suggest): wire autosuggest into the search box behind a flag

Off means off — no combobox role, no listener, no fetch. A test asserts
the absence of the request, not just the absence of the dropdown,
because a hidden-but-fetching control would still be spending the rate
limit on a feature nobody can see.

Enter with no active option falls through to the form's submit handler
and searches the typed text exactly as before. The existing behaviour is
preserved, not replaced, and that has its own test.

Suppressed once the value parses as a postcode: the box takes a name OR
a postcode, and suggesting schools during postcode entry fights the user.

.omniBoxContainer gains position: relative — the dropdown is absolutely
positioned and without it would have anchored to the page instead.

Four render sites, all wired: page.tsx renders HomeView in the success
path AND the catch fallback, and HomeView renders FilterBar as hero AND
sticky. Missing any one would make the flag silently do nothing
somewhere.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015mWQnpye9F299NVRCCSRvj

backend/flags.py

@@ -50,6 +50,13 @@ REGISTRY: dict[str, Flag] = {
             ),
             added=date(2026, 8, 23),
         ),
+        Flag(
+            name="school_autosuggest",
+            description=(
+                "School name suggestions as you type in the main search box."
+            ),
+            added=date(2026, 8, 26),
+        ),
     )
 }