feat(map): standalone ownship marker toggle, decoupled from Live View (#25)

[DocGerd]

Summary

Implements the ownship-marker half of #25 only, per the user-approved
spec addendum "Addendum 2026-07-22: Ownship marker (standalone) + AIS overlay
(deferred) (#25)" in docs/superpowers/specs/2026-07-14-sail-command-design.md.

The AIS traffic overlay is explicitly DEFERRED — no AIS/WebSocket/MMSI
code is in this PR. That decision (BYOK-aisstream is the only compliant
architecture; gated on user go-ahead + a coverage spot-check) is recorded in
the spec addendum only.

• A GPS boat marker (services/geolocation.ts, components/BoatMarker.tsx,
  both unchanged) previously rendered only inside Live View, gated on
  active && legs.length > 0. This generalizes it into a standalone, opt-in
  feature that renders in any map context (planning, no plan, or Live
  View).
• New Settings.showOwnship: boolean field, default OFF, persisted
  exactly like other settings (IndexedDB via db.ts, generically — no
  db.ts/AppState.tsx code changes were needed).
• New useOwnshipGps.ts hook: subscribes to watchPosition only while the
  toggle is on; a denial/unavailable error claims the SAME one-time hint
  LiveView already used (extracted to lib/gpsHint.ts so both consumers
  share one storage-key claim, never a second/duplicate hint).
• New OwnshipMarker.tsx: a thin presentational component that renders
  BoatMarker (+ its accuracy circle) from the hook's fix. Mounted as an
  always-present sibling of RouteLayer/LiveView inside MapView's
  subtree (mirrors DataLayers.tsx's "always-mounted, plan-independent"
  pattern).
• Dedupe: LiveView.tsx no longer renders BoatMarker itself at all —
  OwnshipMarker is now the only render site for it. This makes "no double
  marker when Live View is active and the toggle is on" a structural
  guarantee rather than a runtime race between two GPS watches. Live View's
  own route-following (active-leg highlight, heading-to-steer,
  distance-to-next-maneuver, ETA drift) is unchanged and still requires a
  plan.
• New "show my position" checkbox in OptionsPanel.tsx (mirrors the existing
  motorEnabled checkbox pattern), with a help paragraph carrying the "aid,
  not a navigation device" caveat and consumer-GPS-accuracy framing.
• i18n: options.showOwnship.label / options.showOwnship.help added to
  BOTH dict.de.ts/dict.en.ts (MsgKey parity via satisfies).
• Fully offline: GPS is device-local, no network/key involved.

Test plan

• npm --prefix app run typecheck — clean
• npm --prefix app run lint — clean
• npm --prefix app run test — full suite green: 66 files / 671 tests
  passed (includes realmask.repro.test.ts, unaffected by this change)
• New/updated unit tests (literal-pinned, not derived from the code under
  test):
  • useOwnshipGps.test.ts — does not subscribe while disabled; subscribes
    and exposes the emitted fix verbatim while enabled; unsubscribes +
    clears the fix when disabled again; claims the shared hint on
    denial/unavailable; does NOT re-show the hint if another consumer
    already claimed it; dismissHint hides without re-touching storage.
  • OwnshipMarker.test.tsx (BoatMarker mocked) — renders nothing when
    fix is null; renders BoatMarker with the fix's fields forwarded
    verbatim when present; falls back headingToSteerDeg to 0 absent a
    device COG.
  • LiveView.test.tsx — new test asserts BoatMarker (mocked) is never
    called, even with Live active and a fully steerable fix — the concrete
    proof backing the "no duplicate marker" structural claim above.
  • OptionsPanel.test.tsx — checkbox unchecked against DEFAULT_SETTINGS;
    toggling calls onChange with showOwnship: true immediately; help
    text carries the "not a navigation device" caveat.
  • AppState.test.tsx — showOwnship defaults to 'false'; a patch
    persists to IndexedDB and survives a provider remount.
  • gpsHint.test.ts — claimGpsHintOnce() pinned against the literal
    sc-gps-hint-shown storage key LiveView's own tests already assert
    against (proves the two consumers share one claim, not two).
• Real-browser smoke (dev server, port 5177, Playwright MCP):
  • "Show my position" checkbox present in Settings → Erweitert, unchecked
    by default.
  • Toggled on with a stubbed navigator.geolocation → .sc-boat-marker
    appears on the map with no plan loaded, correctly rotated to the
    fake COG. Screenshot confirms visually.
  • Toggled back off → marker disappears (querySelector returns null).
  • Planned a real route (Langballigau → Sønderborg, deterministic wind
    fixture) and activated Live View with the toggle OFF: a fix arrives, but
    confirmed zero .sc-boat-marker elements (proves Live View alone no
    longer renders one).
  • The Live-active + toggle-ON combined check (proving exactly one marker)
    was not completed live: the shared Playwright MCP browser got
    contended by another concurrent session partway through (page URL
    silently jumped to a different session's localhost:5173, twice — the
    documented shared-browser risk in CLAUDE.md). I did not force it further
    to avoid polluting another session's state. The structural guarantee
    (LiveView never renders BoatMarker — proven above) plus the two clean
    real-browser checks already obtained cover this in spirit; a final
    live double-check of that exact combination before merge would close
    the gap if the browser is free.
  • No console errors (only benign headless-GPU WebGL performance warnings,
    unrelated to this change).
• Wind fixture churn from the browser pass restored
  (git status clean before commit).

Deviations / notes

• None from the spec addendum. geolocation.ts and BoatMarker.tsx are
  byte-unchanged, as required.
• db.ts/AppState.tsx required NO code changes — Settings persistence
  is fully generic (plain-object IndexedDB roundtrip + Partial<Settings>
  merge), so the new field "just works" through existing machinery.
• advancedSummary's one-line "Advanced" recap (PlannerPanel.tsx) was left
  untouched — it exists to recap routing-relevant settings, and
  showOwnship doesn't affect route calculation.

Part of #25.

[DocGerd]

Self-review — approved (one deferred minor)

Reviewed with independent re-verification (tsc -b --noEmit clean, eslint src clean, all 8 touched/new test files re-run: 61/61 pass in the PR's worktree at 9ce55ae).

Minor, non-blocking (deferred to a follow-up): when settings.showOwnship is on AND Live View tracking is simultaneously active, two independent navigator.geolocation.watchPosition subscriptions run concurrently (useOwnshipGps + Live View's own) — extra GPS/battery chatter, no functional bug (single BoatMarker render site verified structurally; no double marker), not required by the spec addendum. Consolidating to a shared GPS provider would touch Live View's working route-following, so it's deferred rather than risked here.

Everything else verified clean: single BoatMarker render site (OwnshipMarker.tsx only; LiveView.tsx no longer imports it); Live View route-following byte-identical apart from marker removal; Settings.showOwnship default false, persists through IndexedDB; services/geolocation.ts + BoatMarker.tsx byte-unchanged; MsgKey parity holds; "aid, not a navigation device" caveat retained; new tests pin literals independent of the code under test. AIS is deliberately deferred (spec-recorded) — this PR is Part of #25, not Closes.