feat(ui): add /dash route, shared nav, project dashboard with /v1/projects API

Restructure routes: / and /dash show a project overview dashboard,
/activity hosts the existing timeline, /cv remains. Shared Layout
component provides consistent nav header and footer across all routes.

New /v1/projects endpoint aggregates per-repo activity stats (commits,
issues, PRs, date range) from existing event data via SQL. Dashboard
ranks projects by weighted recency + volume score and renders a card
grid.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/moments-entities/src/lib.rs

@@ -82,6 +82,19 @@ pub struct SourceSummary {
     pub latest: Option<DateTime<Utc>>,
 }
 
+/// Per-repo activity rollup for the dashboard.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ProjectSummary {
+    pub repo: String,
+    pub source: Source,
+    pub host: String,
+    pub commit_count: i64,
+    pub issue_count: i64,
+    pub pr_count: i64,
+    pub first_activity: Option<DateTime<Utc>>,
+    pub last_activity: Option<DateTime<Utc>>,
+}
+
 // ---------------------------------------------------------------------
 // Presentation shape — what `GET /v1/events` actually returns.
 // The API reshapes raw payloads into these so the frontend stays dumb.