Merge upstream/main: KartaView provider meets i18n

Upstream's KartaView PR (#1121) extracted the Mapillary coverage engine
into a shared lineCoverage.js and made the panel wording
provider-specific (coverage noun/action, SINCE upload-date note,
highlightsSequence legend gate), again in the regions this branch
localizes.

- mapillary/coverage.js: taken as upstream's thin re-export of the new
  lineCoverage.js; the generic too-high hint stays msg()-marked, now in
  lineCoverage.js where the default lives.
- kartaview/policy.js: coverage hints (tooHigh, noCoverage) marked as
  msg() data so they translate like every other coverage hint; thrown
  error messages stay English, matching the Mapillary precedent.
- presentMeta: KartaView counts streets, and Russian plurals need the
  noun inside the template, so the count line gets its own literal tn
  pair keyed off coverageLabel.noun, falling back to the sequences pair;
  upstream's noun/action data contract is untouched.
- uiState legend: upstream highlightsSequence gate with t('Selected');
  sinceNote localized with a {names} placeholder.
- layer-panels.html: sl-since-note wiring kept, data-i18n hooks kept.
- CHANGELOG: both sides kept. es/ru: 4 new keys each.

npm run i18n complete (953 messages each); npm test 6504/6506 (the one
failure is the pre-existing environmental OPENAI_API_KEY leak in
devFreshDotenv, unrelated); build passes.

src/ui/styles/street-level.css

@@ -4,6 +4,9 @@
 #street-level-panel,
 .sl-viewer-wrap {
   --sl-green: #05cb63;
+  /* KartaView's purple for lines; a lighter one where it is text (WCAG AA). */
+  --sl-kartaview: #635bff;
+  --sl-kartaview-text: #8f8aff;
   --sl-text: 0.62rem;
   --sl-text-small: 0.6rem;
 }
@@ -174,6 +177,11 @@ button.sl-status:focus-visible {
   );
 }
 
+/* #635bff text on its own tint is 3.7:1 at this size; the lighter one is ~6:1. */
+.sl-chips .data-toggle-chip.active[data-chip-id='kartaview'] {
+  color: var(--sl-kartaview-text);
+}
+
 .sl-chips .data-toggle-chip.active::before {
   opacity: 1;
 }
@@ -291,6 +299,17 @@ button.sl-status:focus-visible {
   outline-offset: 3px;
 }
 
+.sl-since-note {
+  grid-column: 2;
+  font-family: var(--font-mono);
+  font-size: var(--sl-text-small);
+  color: var(--text-secondary);
+}
+
+.sl-since-note[hidden] {
+  display: none;
+}
+
 .sl-since-label {
   grid-column: 2;
   font-family: var(--font-mono);
@@ -613,6 +632,97 @@ button.sl-status:focus-visible {
   font-size: 1.3rem;
 }
 
+/* ── KartaView photo viewer ──────────────────────────────────────────────── */
+.sl-kv-viewer {
+  position: absolute;
+  inset: 0;
+  background: #000;
+  outline: none;
+  overflow: hidden;
+}
+
+.sl-kv-viewer:focus-visible {
+  outline: none;
+}
+
+/* The photo and canvas fill the viewer and would cover a ring drawn on it:
+   draw the ring on a layer above them (clicks pass through). */
+.sl-kv-viewer:focus-visible::after {
+  content: '';
+  position: absolute;
+  inset: 0;
+  z-index: 3;
+  border: 2px solid var(--sl-kartaview-text);
+  pointer-events: none;
+}
+
+.sl-kv-photo,
+.sl-kv-pano {
+  position: absolute;
+  inset: 0;
+  width: 100%;
+  height: 100%;
+  user-select: none;
+}
+
+.sl-kv-photo {
+  object-fit: contain;
+}
+
+.sl-kv-pano {
+  cursor: grab;
+  touch-action: none;
+}
+
+.sl-kv-pano.sl-kv-dragging {
+  cursor: grabbing;
+}
+
+.sl-kv-photo[hidden],
+.sl-kv-pano[hidden] {
+  display: none;
+}
+
+.sl-kv-step {
+  position: absolute;
+  top: 50%;
+  z-index: 2;
+  width: 34px;
+  height: 44px;
+  margin-top: -22px;
+  border: 1px solid rgba(255, 255, 255, 0.18);
+  border-radius: 8px;
+  background: rgba(10, 10, 15, 0.62);
+  color: #fff;
+  font-size: 1.5rem;
+  line-height: 1;
+  cursor: pointer;
+}
+
+.sl-kv-step-prev {
+  left: 6px;
+}
+
+.sl-kv-step-next {
+  right: 6px;
+}
+
+.sl-kv-step:hover:not(:disabled),
+.sl-kv-step:focus-visible {
+  background: color-mix(in srgb, var(--sl-kartaview) 85%, transparent);
+  outline: none;
+}
+
+.sl-kv-step:disabled {
+  opacity: 0.3;
+  cursor: default;
+}
+
+/* Nothing to step through until a photo shows. */
+.sl-kv-viewer:not([data-kind='flat']):not([data-kind='pano']) .sl-kv-step {
+  display: none;
+}
+
 /* ── Short viewports ─────────────────────────────────────────────────────── */
 @media (max-height: 640px) {
   .street-level-panel-inner {