feat(dashboard): tab strip + Tiles view + view-switcher URL routing

Phase 5h slice 2 — adds the three-tab dashboard chrome (Tiles / Tasks /
Events) and lands the Tiles view as the default landing surface per
m's §7 pick.

URL contract:
  /dashboard                — Tiles (default, elided)
  /dashboard?view=tasks     — today's 5-card layout
  /dashboard?view=events    — Events card promoted to a full-tab view
  Unknown ?view= falls back to Tiles.

Refactor: aggregator calls (Todos / Events / Issues) hoisted up into
buildDashboard so the rollup can consume the same uncapped rows without
a second DAV/Gitea round-trip. The legacy collect* helpers split into
pure projectTasks / projectEvents / projectIssues / projectDocs that
take pre-fetched rows. collectStale extended to return its per-item
repo-activity map alongside the trimmed stale list — the rollup uses
the map as a LastActivity signal.

Cache: key now composes (filter | view=X) so each tab has its own 60s
TTL slot. Tab switches don't poison the cache for siblings.

Tiles render with: pin star (when pinned), title + path + live badge,
counts row (open / overdue! / issues / quiet), NextSignal one-liner
(task wins over issue), and a tile-foot LastActivity stamp.

CSS:
- .dash-tabs strip with active-state border bridge.
- .dash-tiles grid: 1/2/3 cols at 600/900px breakpoints.
- .dash-events-view scaffolding for the promoted Events surface.

Templates: dashboard_section.tmpl restructured to dispatch by .View.
The cards layout is now {{define "dashboard-cards"}} and the
events-only surface is {{define "dashboard-events-view"}}. New
dashboard_tiles.tmpl defines {{define "dashboard-tiles"}}. Both
templates registered in the dashboard + dashboard_section bundles.

Tests:
- Existing dashboard tests retargeted at ?view=tasks for the legacy
  Tasks-tab expectations (5-card layout, inline writeback, stale card).
- New dashboard_view_test.go covers: default view = Tiles, three-tab
  strip rendering + active marker, view=tasks fallback, view=events
  promotion, unknown view fallback, tile rendering for seeded item,
  cache-key separation between views.
- TestLayoutNoTopHeader scoped to the body chrome before <main> so it
  no longer trips on legitimate <header> elements inside cards/tiles.

Out of scope (later slices): scope chip + Quiet fold (slice 3), pin
toggle handler (slice 4), Events tab dedicated polish (slice 5),
mobile polish (slice 7), design.md addendum (slice 8).

web/server.go

@@ -242,17 +242,23 @@ func New(s *store.Store, logger *slog.Logger) (*Server, error) {
 	}
 	pages["admin"] = adminTmpl
 
-	// Dashboard page + its section fragment.
+	// Dashboard page + its section fragment. Phase 5h: the section fragment
+	// dispatches to one of three view partials (tiles / cards / events-view),
+	// so the tiles partial joins both bundles.
 	dashTmpl, err := template.New("dashboard").Funcs(funcs).ParseFS(templatesFS,
 		"templates/layout.tmpl",
 		"templates/dashboard.tmpl",
 		"templates/dashboard_section.tmpl",
+		"templates/dashboard_tiles.tmpl",
 	)
 	if err != nil {
 		return nil, fmt.Errorf("parse dashboard: %w", err)
 	}
 	pages["dashboard"] = dashTmpl
-	dashSection, err := template.New("dashboard_section").Funcs(funcs).ParseFS(templatesFS, "templates/dashboard_section.tmpl")
+	dashSection, err := template.New("dashboard_section").Funcs(funcs).ParseFS(templatesFS,
+		"templates/dashboard_section.tmpl",
+		"templates/dashboard_tiles.tmpl",
+	)
 	if err != nil {
 		return nil, fmt.Errorf("parse dashboard_section: %w", err)
 	}