Refresh the active evidence band live and drop its duplicate switch count

The active view's fast-path repainted only the status band, so the "now"
window, per-window buckets, and their times froze at the moment the active
screen first rendered — only the status-band switch count and the countdown
kept moving. Give the evidence band a stable id and repaint it every tick via
updateActiveEvidence, mirroring updateActiveDrift.

The evidence band's own "context switches" readout duplicated the live count
in the status band, so remove it (and its now-dead .switches CSS rule).

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

internal/web/static/app.css

@@ -98,7 +98,6 @@ input:focus { outline: 0; border-color: var(--accent); }
   display: flex; justify-content: space-between; padding: 2px 0;
   font-family: ui-monospace, monospace; font-variant-numeric: tabular-nums;
 }
-.switches { font-size: 12px; color: var(--ink-dim); margin-top: 8px; }
 
 /* Review summary band */
 .summary { font-size: 14px; }