Graph node FPS hover tooltip, full names, no native SVG tooltips

Graph editor:
- Floating FPS tooltip on hover over running output_target nodes (300ms delay)
- Shows errors, uptime, and FPS sparkline seeded from server metrics history
- Tooltip positioned below node with fade-in/out animation
- Uses pointerover/pointerout with relatedTarget check to prevent flicker
- Fixed-width tooltip (200px) with monospace values to prevent layout shift
- Node titles show full names (removed truncate), no native SVG <title> tooltips

Documentation:
- Added duration/numeric formatting conventions to contexts/frontend.md
- Added node hover tooltip docs to contexts/graph-editor.md

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/src/wled_controller/static/css/graph-editor.css

@@ -1172,3 +1172,62 @@ html:has(#tab-graph.active) {
     background: var(--border-color);
     margin: 4px 0;
 }
+
+/* ── Node hover FPS tooltip ── */
+
+.graph-node-tooltip {
+    position: absolute;
+    z-index: 50;
+    background: var(--card-bg);
+    border: 1px solid var(--border-color);
+    border-radius: var(--radius-md, 6px);
+    box-shadow: 0 4px 14px var(--shadow-color, rgba(0,0,0,0.25));
+    padding: 8px 12px;
+    pointer-events: none;
+    font-size: 0.8rem;
+    width: 200px;
+    color: var(--text-color);
+}
+
+.graph-node-tooltip .gnt-row {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    gap: 12px;
+    line-height: 1.6;
+}
+
+.graph-node-tooltip .gnt-label {
+    color: var(--text-muted);
+    white-space: nowrap;
+}
+
+.graph-node-tooltip .gnt-value {
+    font-variant-numeric: tabular-nums;
+    font-weight: 500;
+    text-align: right;
+    min-width: 72px;
+    display: inline-block;
+    font-family: var(--font-mono, 'Consolas', 'Monaco', monospace);
+}
+
+.graph-node-tooltip .gnt-fps-row {
+    margin-top: 4px;
+    padding: 2px 0;
+    background: transparent;
+}
+
+.graph-node-tooltip.gnt-fade-in {
+    animation: gntFadeIn 0.15s ease-out forwards;
+}
+.graph-node-tooltip.gnt-fade-out {
+    animation: gntFadeOut 0.12s ease-in forwards;
+}
+@keyframes gntFadeIn {
+    from { opacity: 0; transform: translateY(-4px); }
+    to   { opacity: 1; transform: translateY(0); }
+}
+@keyframes gntFadeOut {
+    from { opacity: 1; transform: translateY(0); }
+    to   { opacity: 0; transform: translateY(4px); }
+}