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>

contexts/frontend.md

@@ -219,6 +219,31 @@ When adding a new JS dependency: `npm install <pkg>` in `server/`, then `import`
 
 See [`contexts/chrome-tools.md`](chrome-tools.md) for Chrome MCP tool usage, browser tricks (hard reload, zoom, console), and verification workflow.
 
+## Duration & Numeric Formatting
+
+### Uptime / duration values
+
+Use `formatUptime(seconds)` from `core/ui.js`. Outputs `{s}s`, `{m}m {s}s`, or `{h}h {m}m` via i18n keys `time.seconds`, `time.minutes_seconds`, `time.hours_minutes`.
+
+### Large numbers
+
+Use `formatCompact(n)` from `core/ui.js`. Outputs `1.2K`, `3.5M` etc. Set `element.title` to the exact value for hover detail.
+
+### Preventing layout shift
+
+Numeric/duration values that update frequently (FPS, uptime, frame counts) **must** use fixed-width styling to prevent layout reflow:
+
+- `font-family: var(--font-mono, monospace)` — equal-width characters
+- `font-variant-numeric: tabular-nums` — equal-width digits in proportional fonts
+- Fixed `width` or `min-width` on the value container
+- `text-align: right` to anchor the growing edge
+
+Reference: `.dashboard-metric-value` in `dashboard.css` uses `font-family: var(--font-mono)`, `font-weight: 600`, `min-width: 48px`.
+
+### FPS sparkline charts
+
+Use `createFpsSparkline(canvasId, actualHistory, currentHistory, fpsTarget)` from `core/chart-utils.js`. Wrap the canvas in `.target-fps-sparkline` (36px height, `position: relative`, `overflow: hidden`). Show the value in `.target-fps-label` with `.metric-value` and `.target-fps-avg`.
+
 ## Visual Graph Editor
 
 See [`contexts/graph-editor.md`](graph-editor.md) for full graph editor architecture and conventions.