fix(hikes): repair Swisstopo elevation API (LV95 + POST), add busy chip

Two upstream constraints were causing the densified route-builder
tracks to ship without `<ele>` on most points:

1. api3.geo.admin.ch's elevation services reject `sr=4326` outright
   (HTTP 400: "Please provide a valid number for the spatial
   reference system model: 21781, 2056"). Add an in-process
   WGS84→LV95 converter using Swisstopo's published approximation
   (≈1 m positional accuracy, well below the DTM grid resolution)
   and switch both `height` and `profile.json` to sr=2056.

2. profile.json GET silently 414s once the URL crosses ~8 KB. At
   our densified-track sizes a 200-coord chunk hit ~9.6 KB and got
   dropped — that's why only a handful of segments came back with
   elevations. Switch to POST + form-encoded body; chunk size can
   safely go to 500 coords.

UX: extend the busy state to cover the densify+elevate path
(previously only set during snap-to-route) and add a mode-agnostic
status chip in the header that pulses amber while the elevation
request is in flight. GPX download button is now disabled while
busy so the file can't be exported half-finished.

src/routes/hikes/route-builder/+page.svelte

@@ -117,8 +117,15 @@
 				// overwrite what we're about to densify.
 				const reqId = ++routeRequestId;
 				snapDebounce = setTimeout(async () => {
-					densifyLinearSegments(25);
-					await enrichMissingElevations(reqId);
+					busy = true;
+					try {
+						densifyLinearSegments(25);
+						await enrichMissingElevations(reqId);
+					} catch (err) {
+						if (reqId === routeRequestId) error = (err as Error).message;
+					} finally {
+						if (reqId === routeRequestId) busy = false;
+					}
 				}, 250);
 			}
 		});
@@ -230,9 +237,22 @@
 			</select>
 			<label class="snap-toggle" class:active={builder.autoSnap}>
 				<input type="checkbox" bind:checked={builder.autoSnap} />
-				<span>Auf Wege snappen{busy ? ' …' : ''}</span>
+				<span>Auf Wege snappen</span>
 			</label>
-			<button type="button" class="primary" onclick={downloadGpx}>
+			<!-- Mode-agnostic busy chip — fires for both the snap-to-route
+			     path and the densify+elevate path so the user always knows
+			     when the GPX is still incomplete. -->
+			<span class="status" class:busy aria-live="polite" aria-atomic="true">
+				<span class="status-dot" aria-hidden="true"></span>
+				{busy ? 'Berechne Route + Höhenprofil…' : 'Bereit'}
+			</span>
+			<button
+				type="button"
+				class="primary"
+				onclick={downloadGpx}
+				disabled={busy}
+				title={busy ? 'Warten bis Route + Höhenprofil berechnet sind' : ''}
+			>
 				GPX herunterladen
 			</button>
 			<button type="button" class="link" onclick={clearDraft}>Zurücksetzen</button>
@@ -359,6 +379,56 @@
 		height: 1rem;
 	}
 
+	/* Live busy chip — sits between the snap toggle and the download
+	 * button so the user can't miss it when GPX export would land
+	 * without elevations yet. Quiet green dot when idle, pulsing
+	 * amber dot when fetching. */
+	.status {
+		display: inline-flex;
+		align-items: center;
+		gap: 0.4rem;
+		font-size: 0.8rem;
+		color: var(--color-text-secondary);
+		font-variant-numeric: tabular-nums;
+		min-width: 0;
+	}
+
+	.status-dot {
+		width: 0.55rem;
+		height: 0.55rem;
+		border-radius: 50%;
+		background: var(--green);
+		flex: 0 0 auto;
+		box-shadow: 0 0 0 0 color-mix(in oklab, var(--green) 40%, transparent);
+	}
+
+	.status.busy {
+		color: var(--orange);
+	}
+
+	.status.busy .status-dot {
+		background: var(--orange);
+		animation: status-pulse 1.1s ease-out infinite;
+	}
+
+	@keyframes status-pulse {
+		0% {
+			box-shadow: 0 0 0 0 color-mix(in oklab, var(--orange) 70%, transparent);
+		}
+		70% {
+			box-shadow: 0 0 0 0.55rem color-mix(in oklab, var(--orange) 0%, transparent);
+		}
+		100% {
+			box-shadow: 0 0 0 0 color-mix(in oklab, var(--orange) 0%, transparent);
+		}
+	}
+
+	@media (prefers-reduced-motion: reduce) {
+		.status.busy .status-dot {
+			animation: none;
+		}
+	}
+
 	.grid {
 		display: grid;
 		grid-template-columns: minmax(0, 1.5fr) minmax(280px, 1fr);