fix(stammbaum): SVG node font 14→16px and reliable keyboard focus ring

CSS box-shadow rings (focus-visible:ring-*) are invisible inside SVG.
Replace with a conditional <rect> drawn at -3px offset that renders in
all browsers. Name font-size bumped from 14 to 16px for the 60+
transcriber audience (WCAG readability, Leonie medium concerns).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/StammbaumTree.svelte

@@ -286,6 +286,8 @@ function nodeCenter(id: string): { x: number; y: number } | null {
 	return { x: p.x + NODE_W / 2, y: p.y + NODE_H / 2 };
 }
 
+let focusedId = $state<string | null>(null);
+
 function handleNodeKey(event: KeyboardEvent, id: string) {
 	if (event.key === 'Enter' || event.key === ' ') {
 		event.preventDefault();
@@ -479,6 +481,7 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 		{@const pos = layout.positions.get(node.id)}
 		{#if pos}
 			{@const isSelected = selectedId === node.id}
+			{@const isFocused = focusedId === node.id}
 			<g
 				role="button"
 				tabindex="0"
@@ -489,8 +492,22 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 				transform="translate({pos.x}, {pos.y})"
 				onclick={() => onSelect(node.id)}
 				onkeydown={(e) => handleNodeKey(e, node.id)}
-				class="cursor-pointer focus:outline-none focus-visible:ring-2 focus-visible:ring-primary"
+				onfocus={() => (focusedId = node.id)}
+				onblur={() => (focusedId = null)}
+				class="cursor-pointer focus:outline-none"
 			>
+				{#if isFocused}
+					<rect
+						x="-3"
+						y="-3"
+						width={NODE_W + 6}
+						height={NODE_H + 6}
+						rx="6"
+						fill="none"
+						stroke="var(--c-focus-ring)"
+						stroke-width="2"
+					/>
+				{/if}
 				<rect
 					width={NODE_W}
 					height={NODE_H}
@@ -504,10 +521,10 @@ const parentLinks = $derived.by<ParentLinks>(() => {
 				{/if}
 				<text
 					x={NODE_W / 2}
-					y={NODE_H / 2 - 4}
+					y={NODE_H / 2 - 6}
 					text-anchor="middle"
 					font-family="serif"
-					font-size="14"
+					font-size="16"
 					fill={isSelected ? 'var(--c-primary-fg)' : 'var(--c-ink)'}
 				>
 					{node.displayName}