feat(stammbaum): bottom-right zoom + fit-to-screen control cluster (#692)

Move zoom controls out of the page header into a docked bottom-right cluster
inside the canvas (one-handed phone reach, Leonie) and add a fit-to-screen
button (data-testid=fit-to-screen). Add the 5 new i18n keys to de/en/es.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/StammbaumControls.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+import { m } from '$lib/paraglide/messages.js';
+
+interface Props {
+	onZoomIn: () => void;
+	onZoomOut: () => void;
+	onFit: () => void;
+}
+
+let { onZoomIn, onZoomOut, onFit }: Props = $props();
+
+// Docked bottom-right inside the canvas — the primary one-handed reach zone on a
+// phone (Leonie). The container ignores pointer events so canvas gestures pass
+// through the gaps; only the buttons capture taps.
+const buttonClass =
+	'pointer-events-auto inline-flex h-11 w-11 items-center justify-center rounded-sm border border-line bg-surface text-lg text-ink-2 shadow-sm transition hover:bg-muted focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none';
+</script>
+
+<div
+	class="pointer-events-none absolute right-4 z-30 flex flex-col gap-1"
+	style="bottom: max(calc(env(safe-area-inset-bottom, 0px) + 1rem), 1rem);"
+>
+	<button type="button" onclick={onZoomIn} aria-label={m.stammbaum_zoom_in()} class={buttonClass}>
+		+
+	</button>
+	<button type="button" onclick={onZoomOut} aria-label={m.stammbaum_zoom_out()} class={buttonClass}>
+		−
+	</button>
+	<button
+		type="button"
+		data-testid="fit-to-screen"
+		onclick={onFit}
+		aria-label={m.stammbaum_fit_to_screen()}
+		class={buttonClass}
+	>
+		⤢
+	</button>
+</div>