feat(ui): contribution graph with daily activity heatmap

Add /v1/activity/daily endpoint returning per-day event counts via
generate_series + LEFT JOIN. Frontend renders an SVG contribution
graph with circles colored by quantile-based thresholds. Clicking a
day navigates to /activity/YYYY-MM-DD showing that day's events.

New /activity/:timespan route parses single dates (YYYY-MM-DD) and
ranges (YYYY-MM-DD..YYYY-MM-DD) from the URL to initialize the
activity timeline filter.

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

crates/moments-data/src/lib.rs

@@ -8,7 +8,8 @@ pub mod hg;
 use async_trait::async_trait;
 use chrono::{DateTime, Utc};
 use moments_core::{EventReader, EventWriter, PollerState, PollerStateStore, StoreError};
-use moments_entities::{Event, EventQuery, ProjectSummary, Source, SourceSummary};
+use chrono::NaiveDate;
+use moments_entities::{DailyCount, Event, EventQuery, ProjectSummary, Source, SourceSummary};
 use sqlx::Row;
 use sqlx::postgres::{PgPool, PgPoolOptions};
 use std::str::FromStr;
@@ -192,6 +193,35 @@ impl EventReader for PgStore {
             })
             .collect()
     }
+
+    async fn daily_counts(&self, from: NaiveDate, to: NaiveDate) -> Result<Vec<DailyCount>, StoreError> {
+        let rows = sqlx::query(
+            r#"
+            SELECT d::date AS date,
+                   COUNT(e.id)::bigint AS count
+            FROM generate_series($1::date, $2::date, '1 day') d
+            LEFT JOIN events e
+              ON e.occurred_at >= d AND e.occurred_at < d + interval '1 day'
+              AND e.public = true
+            GROUP BY d::date
+            ORDER BY d::date
+            "#,
+        )
+        .bind(from)
+        .bind(to)
+        .fetch_all(&self.pool)
+        .await
+        .map_err(map_err)?;
+
+        rows.into_iter()
+            .map(|r| {
+                Ok(DailyCount {
+                    date: r.try_get("date").map_err(map_err)?,
+                    count: r.try_get("count").map_err(map_err)?,
+                })
+            })
+            .collect()
+    }
 }
 
 #[async_trait]