Switch read paths to the true-instant date contract (ticket #119)

- New src/date_view.rs: pure read-side functions (row_true_instant,
  row_display_wall, row_display_day, filename_true_instant) applying the
  per-source convention table for unconverted rows and the new-contract
  triple for converted rows (ADR-0001).
- /memories: grouping on the row's true instant rendered in the owner's
  display zone (prefs -> app default), not the client device offset.
  SQL is a coarse interval pre-filter over the full years-back window;
  exact month-day/week/month matching across past years happens in Rust
  (memories::day_in_span, sqlite_w_week).
- Reels: ReelSelector::Memories carries an IANA display_tz instead of a
  fixed minute offset; pre-gen params read REEL_PREGEN_DISPLAY_TZ; the
  tz_offset_minutes ledger column stores the zone offset at render time.
- Insights: prompt-facing dates render as the display wall; the
  proximity timestamp and chat context block use the true instant;
  InsightGenerator reads the owner's display zone from prefs per call.
- GpsPhotoSummary carries the new-contract triple alongside the frozen
  legacy date_taken.

Closes #119

src/data/mod.rs

@@ -428,7 +428,15 @@ pub struct GpsPhotoSummary {
     pub path: String,
     pub lat: f64,
     pub lon: f64,
+    /// Legacy mixed-convention date (frozen for existing rows; ADR-0001).
     pub date_taken: Option<i64>,
+    /// True instant (epoch seconds), when the row has been converted.
+    pub date_utc: Option<i64>,
+    /// Camera offset in minutes from UTC at capture, when known.
+    pub camera_offset_minutes: Option<i32>,
+    /// Which evidence pinned the instant (`offset_tag`, `gps`,
+    /// `assumed_server`, `utc_direct`, `manual`).
+    pub date_taken_tz_source: Option<String>,
 }
 
 #[derive(Debug, Serialize)]