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/docs/error-state-ux.md

@@ -0,0 +1,51 @@
+# Error & state UX
+
+How the client gives consistent, actionable feedback for failures and for
+loading / empty / error states.
+
+## Error surface (`src/lib/error/`)
+
+Server `result_code`s are downstream-opaque (only `"ok"` is contractual —
+`docs/ARCHITECTURE.md`), so the client keys UX off the reliable
+transport-level signal instead.
+
+- [`classify.ts`](../frontend/src/lib/error/classify.ts) collapses any
+  caught value into a stable `ErrorKind` (`offline`, `network`, `auth`,
+  `forbidden`, `conflict`, `notFound`, `rateLimit`, `server`, `unknown`)
+  from `navigator.onLine`, an `AuthError` HTTP status, a Connect `Code`,
+  or a fetch `TypeError`. `isRetryable` marks the transient kinds.
+- [`report.ts`](../frontend/src/lib/error/report.ts) maps each kind to a
+  translated `error.*` message and either:
+  - `reportError(err, { onRetry? })` — raises a toast (a sticky toast
+    with a **Retry** action for a retryable kind that supplies `onRetry`,
+    otherwise auto-dismiss); or
+  - `errorMessageKey(err)` — returns the translated key for a view to
+    render the message inline (e.g. the mail compose 403).
+
+Messages live under `error.*` (en + ru); the action label is
+`common.retry`. New call sites adopt this surface incrementally; the mail
+compose dialog is the worked inline example.
+
+## View states (`src/lib/ui/view-state.svelte`)
+
+`<ViewState kind="loading|empty|error" message=… [actionLabel onAction] />`
+is the one placeholder for a view or panel: a spinner for loading, an
+accent action button when given, and the right live-region role
+(`status` for loading/empty, `alert` for error). The entity tables
+(races / sciences / ship-classes) use it; remaining views adopt it
+incrementally.
+
+## Selected-planet marker (`src/map/selection-ring.ts`)
+
+When the `SelectionStore` holds a planet, the map draws one accent ring
+tight around it (`computeSelectionRing`, fed into the map's `buildExtras`
+alongside the reach circles). Ship-group selection is not ringed — groups
+are addressed by report index and have no single stable map coordinate.
+
+## Mobile bottom-sheet dismissal (`src/lib/ui/sheet-dismiss.ts`)
+
+`use:sheetDismiss={{ onDismiss }}` adds two hand-rolled (no-dependency)
+gestures to the planet / ship-group bottom-sheets: tap anywhere outside
+the sheet, or drag its `[data-sheet-handle]` grabber down past a
+threshold. The swipe starts only from the grabber so it never fights the
+sheet's own content scrolling.