feat(ui): error & state UX — error surface, view states, map selection, sheet gestures (F4)

- lib/error/: classify any caught error into a stable ErrorKind from the
  transport signal (HTTP status / Connect Code / fetch TypeError /
  navigator.onLine); map to translated error.* messages via reportError
  (sticky Retry toast for retryable kinds) or errorMessageKey (inline).
  Mail compose now surfaces the translated 403/error inline.
- lib/ui/view-state.svelte: shared loading/empty/error placeholder with
  the right live-region role + optional action; entity tables
  (races/sciences/ship-classes) migrated, rest adopt incrementally.
- map/selection-ring.ts: accent ring around the selected planet, fed into
  the map buildExtras alongside the reach circles.
- lib/ui/sheet-dismiss.ts: tap-outside + drag-handle swipe-down dismissal
  for the planet/ship-group bottom-sheets (hand-rolled pointer events).

Tests: error, view-state, selection-ring, sheet-dismiss (761 total).
Docs: ui/docs/error-state-ux.md (+ index); F4 marked done.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ui/frontend/src/map/selection-ring.ts

@@ -0,0 +1,47 @@
+// Selected-planet marker. When the SelectionStore holds a planet, the
+// map draws one accent ring tight around it so the current selection is
+// visible on the canvas itself (the inspector/sheet show the detail).
+// Ship-group selection is intentionally not ringed here — groups are
+// addressed by report index and have no single stable map coordinate.
+
+import type { CirclePrim } from "./world";
+
+/** Planet marker radius in world units; mirrors `battle-markers.ts`. */
+const PLANET_RADIUS_WORLD = 6;
+/** The ring sits just outside the marker (and the bombing ring at +3). */
+const SELECTION_RING_RADIUS = PLANET_RADIUS_WORLD + 4;
+
+export const SELECTION_RING_COLOR = 0x6d8cff;
+/** High-bit prefix so the ring id never collides with planet numbers,
+ * route lines, reach rings (`0xb…`), or battle markers. */
+export const SELECTION_RING_ID = 0xc0000000;
+/** Below interactive primitives so it never wins a click. */
+const SELECTION_RING_PRIORITY = 0;
+
+/**
+ * computeSelectionRing returns one ring primitive centred on the selected
+ * planet, or `null` when nothing (or a non-planet) is selected or the
+ * planet is absent from the current report.
+ */
+export function computeSelectionRing(
+	planets: ReadonlyArray<{ number: number; x: number; y: number }>,
+	selectedPlanetId: number | null,
+): CirclePrim | null {
+	if (selectedPlanetId === null) return null;
+	const planet = planets.find((p) => p.number === selectedPlanetId);
+	if (planet === undefined) return null;
+	return {
+		kind: "circle",
+		id: SELECTION_RING_ID,
+		priority: SELECTION_RING_PRIORITY,
+		hitSlopPx: 0,
+		x: planet.x,
+		y: planet.y,
+		radius: SELECTION_RING_RADIUS,
+		style: {
+			strokeColor: SELECTION_RING_COLOR,
+			strokeAlpha: 0.95,
+			strokeWidthPx: 1.5,
+		},
+	};
+}