fix: inspector stale data warning, remove dead supported_modes code

- inspector.html: show orange '⚠ Stale: HH:MM' with tooltip when link_stats data is >15 min old (previously just showed the time with no visual warning)
- style.css: add .g-stale-warn helper class (orange, bold) for the stale indicator
- diagnose.py: remove supported_modes accumulation from parse_ethtool() — field was collected but never consumed by analyze() or displayed anywhere

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

static/style.css

@@ -217,6 +217,7 @@
 .sev-pills          { display: flex; gap: 4px; }
 .g-page-sub { font-size: .78em; color: var(--text-muted); margin-top: 4px; }
 .g-page-sub-aside { font-size: .78em; color: var(--text-muted); margin-left: 8px; }
+.g-stale-warn { color: var(--orange); font-weight: 600; }
 
 /* ── Badge severity color variants (used with lt-badge) ───────────── */
 .badge-critical { color: var(--red);    border-color: var(--red);    text-shadow: var(--glow-red); }