M2: frontend walkthrough fixes + explicit dev compose stack

Post-M2 self-walkthrough polish, batched into one commit.

Map / heat:
- fix heat-layer white-screen crash after login (add layer to map before
  setLatLngs; an off-map leaflet.heat layer has a null _map and throws)
- normalize each heat layer to the densest pixel cell visible in the CURRENT
  viewport (maxZoom:0 so intensity factor f=1) and recompute on moveend/zoomend,
  so sparse poo data reaches red and stays normalized at any zoom level
- dark CARTO basemap tiles when the color scheme is dark

UI:
- dark-mode toggle in the top-right, beside the settings gear
- switch top-right nav (records / theme / settings / logout) to Feather icons
  with hover tooltips
- home: Grafana-style quick time-range presets + back/forward shift buttons,
  placed between the From/To pickers and Apply; fix Select/tooltip z-index
  (Leaflet stacking) and the shift-button height alignment

API client:
- stop flooding GET /api/session with 401s: the session probe and the login
  endpoint own their 401s (no global redirect), which fixes the logout hang and
  the spinning login page

Compose:
- rename docker-compose.override.yml -> docker-compose.dev.yml as an explicit,
  non-auto-layered dev stack (8001, -dev container names, prod-copy ./data DB);
  update tests/test_deployment.py (read dev.yml, tolerate the !override tag) and
  the README "Docker Compose" section

Tests:
- pixel-grid peak counter, time-range presets, heat-layer ordering regression,
  and 401-redirect regression

frontend/src/map/mapUtils.ts

@@ -40,6 +40,31 @@ export function pooToHeatPoints(records: PooRecord[]): HeatPoint[] {
   return records.map((r) => [r.latitude, r.longitude, 1])
 }
 
+/**
+ * Peak number of 2D coordinates that fall into the same `cellSize`-sized grid
+ * cell. Pure + leaflet-free so it is unit-testable.
+ *
+ * Used by the map heat normalization: project the VISIBLE points to screen
+ * pixels (in the map component), then this returns the densest pixel cell's
+ * count, which becomes leaflet.heat's `max`. With maxZoom:0 (intensity factor
+ * f=1) the accumulated per-cell value equals this count, so the densest visible
+ * cluster maps to the hot color — recomputed on every zoom/pan so it always
+ * normalizes within the current viewport. Returns at least 1.
+ */
+export function peakGridCount(coords: Array<[number, number]>, cellSize: number): number {
+  if (coords.length === 0) return 1
+  const g = Math.max(1, cellSize)
+  const counts = new Map<string, number>()
+  let peak = 1
+  for (const [x, y] of coords) {
+    const key = `${Math.floor(x / g)}:${Math.floor(y / g)}`
+    const next = (counts.get(key) ?? 0) + 1
+    counts.set(key, next)
+    if (next > peak) peak = next
+  }
+  return peak
+}
+
 /**
  * Convert location records to map points (for scatter layer).
  */
@@ -102,3 +127,58 @@ export function computeCenter(
   const sumLng = points.reduce((s, p) => s + p.lng, 0)
   return [sumLat / points.length, sumLng / points.length]
 }
+
+// ---------------------------------------------------------------------------
+// Quick time-range presets + window shifting (Grafana-style)
+// ---------------------------------------------------------------------------
+
+const HOUR_MS = 3_600_000
+const DAY_MS = 24 * HOUR_MS
+
+/** A quick-range preset: a label + a span in milliseconds (month/year approximated). */
+export interface TimePreset {
+  value: string
+  label: string
+  spanMs: number
+}
+
+export const TIME_PRESETS: TimePreset[] = [
+  { value: '24h', label: 'Past 24 hours', spanMs: 24 * HOUR_MS },
+  { value: '1w', label: 'Past 1 week', spanMs: 7 * DAY_MS },
+  { value: '2w', label: 'Past 2 weeks', spanMs: 14 * DAY_MS },
+  { value: '1mo', label: 'Past 1 month', spanMs: 30 * DAY_MS },
+  { value: '6mo', label: 'Past 6 months', spanMs: 182 * DAY_MS },
+  { value: '1y', label: 'Past 1 year', spanMs: 365 * DAY_MS },
+  { value: '5y', label: 'Past 5 years', spanMs: 5 * 365 * DAY_MS },
+]
+
+/** ISO8601 with second precision, no milliseconds: "YYYY-MM-DDTHH:MM:SSZ". */
+function isoSeconds(d: Date): string {
+  return d.toISOString().slice(0, 19) + 'Z'
+}
+
+/**
+ * Compute a [start, end] window of width `spanMs` ending at `now`.
+ * Used when the user picks a quick-range preset.
+ */
+export function presetRange(spanMs: number, now: Date = new Date()): { start: string; end: string } {
+  return { start: isoSeconds(new Date(now.getTime() - spanMs)), end: isoSeconds(now) }
+}
+
+/**
+ * Shift a [start, end] window by its OWN span. direction = -1 moves earlier
+ * (back in time), +1 moves later. The window width is preserved.
+ */
+export function shiftRange(
+  startISO: string,
+  endISO: string,
+  direction: -1 | 1,
+): { start: string; end: string } {
+  const startMs = Date.parse(startISO)
+  const endMs = Date.parse(endISO)
+  const span = endMs - startMs
+  return {
+    start: isoSeconds(new Date(startMs + direction * span)),
+    end: isoSeconds(new Date(endMs + direction * span)),
+  }
+}