feat(ui): base stylesheet on the Steward design-system tokens; readable status colours (#5092)

#5092 "Tokens". base.html's inline palette was its own: white text on the
gold button (1.81:1), a red at 3.26:1 on a card, a warning yellow at hue
42 beside the gold accent at 46, a lavender page title, serif labels at
13px and weights up to 700.

- static/theme.css is design system 6's generated sheet (FabledSword + the
  Steward accent). static/steward.css is base.html's stylesheet moved out,
  with :root now aliases onto those tokens (--bg, --text-muted, --green…)
  so every template keeps rendering; it states no colour of its own.
- Status text colours are Steward overrides recorded in the design system
  (tokens 182-184): success #5FB87A, warning #E08A3C, error #EC7A6C. The
  house -fg mixes read "up" as grey hint text (dE 8.2) and warning as the
  accent (dE 11.5); these are >= 4.5:1 on page, card, hover and their own
  15-25% tints. --orange is now the midpoint of warning and error.
- Primary button and active range button: dark text on gold (7.82:1).
  Danger button on the house destructive fill. Badges and alerts take the
  -fg colour on a tint; the gold badge uses --fs-accent-fg.
- Type: Inter body, Fraunces only for page titles and h1/h2, JetBrains
  Mono for code; section and widget titles become the tiny caps label in
  the body face; weights capped at 500.
- Removed: the lavender title and share-view gradient, #606080, and the
  undefined --card-bg, --accent-faint, --accent-bg, --bg-alt, --border-low
  and --row-alt (now real tokens); the dead settings/index.html.
- Guards (tests/test_style_tokens.py): every var(--x) used is defined;
  steward.css states no colour or weight over 500; hex colours in
  templates may only go down (ceiling 75, from 81 before this change),
  each with a self-test that it can fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

plugins/snmp/templates/snmp/index.html

@@ -31,7 +31,7 @@
     {% for oid_cfg in device.oids %}
     {% set label = oid_cfg.label if oid_cfg.label else oid_cfg.oid %}
     {% set val = device.readings.get(label) %}
-    <div style="background:var(--bg-alt);border-radius:6px;padding:0.6rem 0.9rem;">
+    <div style="background:var(--bg-elevated);border-radius:6px;padding:0.6rem 0.9rem;">
       <div style="font-size:0.72rem;color:var(--text-dim);margin-bottom:0.2rem;font-family:monospace;">{{ label }}</div>
       {% if val is not none %}
       <div style="font-size:1.1rem;font-weight:600;font-variant-numeric:tabular-nums;">

plugins/snmp/templates/snmp/widget.html

@@ -36,7 +36,7 @@
   {% endif %}
 </div>
 {% if not loop.last %}
-<div style="height:1px;background:var(--border-low);margin:0.1rem 0;"></div>
+<div style="height:1px;background:var(--border);margin:0.1rem 0;"></div>
 {% endif %}
 {% endfor %}
 {% if total > devices|length %}