feat(views): Phase 5i slice E — default view-per-page + opt-out banner

Closes the Phase 5i implementation chain. When `views.is_default_for=<page>`
is set, opening that page with a "clean" URL (no chip params, no
?view=) auto-applies the saved filter + view_type. A "Showing default
view: <name> · clear" banner makes the swap visible and gives the user
a one-click out. Adding any chip param to the URL bypasses the default;
?nodefault=1 is the explicit opt-out for "I want the bare default tree".

New web/views.go: applyDefaultView gates on the param-cleanness check
+ Store.DefaultViewFor lookup. Resolution + view_type revalidation
mirror the slice D ?view=<uuid> path so a kanban-default opened on a
route that doesn't allow kanban falls back cleanly.

handleTree wires it into the existing slice D else-branch (no default
when ?view= is set). DefaultBanner field passes the applied view to
the template for the banner.

Test:
- TestDefaultViewAppliedOnCleanURL — seeds a tree default with
  filter_json={tags:[work]} + view_type=card, then asserts: clean GET /
  applies (card grid + banner with the view's name); ?tag=dev bypasses
  (forest, no banner); ?nodefault=1 opt-out (forest, no banner).

web/static/style.css

@@ -259,6 +259,15 @@ table.classify input, table.classify select { width: 100%; }
 .kanban-card-title:hover { color: var(--accent); }
 .kanban-card-meta { display: flex; flex-wrap: wrap; gap: 4px; margin: 4px 0 0; font-size: 0.78em; }
 .kanban-empty { padding: 24px; }
+/* Phase 5i Slice E — default-view banner. Sits above the counts line on
+   any Views-supporting page when a default view is auto-applied. */
+.default-banner {
+  background: var(--surface); border: 1px solid var(--border);
+  border-left: 3px solid var(--accent);
+  padding: 6px 10px; border-radius: 4px;
+  font-size: 0.85em; margin: 4px 0 8px;
+}
+.default-banner a { color: var(--bad); }
 #tree-filterbar small { opacity: 0.75; margin-left: 2px; }
 .tree-section .empty { padding: 24px; color: var(--muted); }
 .tree-section .clear { color: var(--bad); }