Lattices: replace index tab bar with collapsible sidebar submenu

Reviewer-requested changes on the lattice section revamp:

- The site has moved from per-section tab bars to collapsible sidebar
  submenus, so give the Lattices sidebar entry a submenu (lattices,
  genera) following the Abstract groups pattern, drop the tabs macro
  from lattice-index.html and genus-index.html, and delete
  lattice-tabs.html.
- Reword the two Eisenstein newspace comments in web_lattice.py: the
  Eisenstein branch IS part of alpha; the try/except is there so the
  lattices branch can also be deployed without it.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: roed314

lmfdb/homepage/sidebar.yaml

@@ -211,6 +211,12 @@
     url_for: "lattice.index"
     status: beta
     colspan: onecolumn
+    collapsible: true
+    part2:
+      - title: "lattices"
+        url_for: "lattice.index"
+      - title: "genera"
+        url_for: "lattice.genus_index"
   - title: Finite subgroups of
     part2:
       - title: $\GL(n,\F_q)$

lmfdb/lattice/templates/genus-index.html

@@ -1,8 +1,5 @@
 {% extends "homepage.html" %}
 {% block content %}
-{% from "lattice-tabs.html" import tabs %}
-
-{{ tabs(("Lattices", ".index", false), ("Genera", ".genus_index", true)) }}
 
 <div>
 {{ info.stats.short_summary | safe }}

lmfdb/lattice/templates/lattice-index.html

@@ -1,8 +1,5 @@
 {% extends "homepage.html" %}
 {% block content %}
-{% from "lattice-tabs.html" import tabs %}
-
-{{ tabs(("Lattices", ".index", true), ("Genera", ".genus_index", false)) }}
 
 <div>
 {{ info.stats.short_summary | safe }}

lmfdb/lattice/templates/lattice-tabs.html

@@ -355,8 +355,9 @@ def friends(self):
         chi_orbit = 'a' if chi.is_trivial() else 'b'
         mf_space = f'{self.level}.{weight}.E.{chi_orbit}'
         try:
-            # Eisenstein newspace labels are only supported once the CMF
-            # Eisenstein branch is merged; skip the friend if not available.
+            # The CMF code accepts Eisenstein (E) newspace labels only when
+            # the Eisenstein branch is present (it is on alpha); degrade
+            # gracefully so the lattices branch can also be deployed without it.
             friends.append(("Newspace " + mf_space, url_for_mf_label(mf_space)))
         except ValueError:
             pass
@@ -545,8 +546,9 @@ def friends(self):
         chi_orbit = 'a' if chi.is_trivial() else 'b'
         mf_space = f'{self.level}.{weight}.E.{chi_orbit}'
         try:
-            # Eisenstein newspace labels are only supported once the CMF
-            # Eisenstein branch is merged; skip the friend if not available.
+            # The CMF code accepts Eisenstein (E) newspace labels only when
+            # the Eisenstein branch is present (it is on alpha); degrade
+            # gracefully so the lattices branch can also be deployed without it.
             friends.append(("Newspace " + mf_space, url_for_mf_label(mf_space)))
         except ValueError:
             pass