Neutral graphite theme; branded updater icon

Coloring: the base surfaces were green-tinted darks, which read as a
green-hued background rather than green highlights. Shift the whole
surface ramp to neutral graphite (slightly cool, and lighter overall)
and keep green strictly for highlights — brand, buttons, switches,
status dots and pills, sync bars, focus rings.

- Tokens: bg #17191d, surface #1c1f24, card #23272c, hover #292e34,
  elevated #26292e, inset #121417; text neutrals #e9edec/#a9b0b3/
  #7a8388. Accent, borders, radii, shadows unchanged.
- Remove the green ambient radial washes behind the app (and login).
- Reboot / security-reset overlay gradients neutralized.

Updater icon: restore the repo's branded Sovran updater icon
(/static/icons/update.svg, true colors) in the sidebar Update System
row and in the update dialog header, replacing the generic refresh
glyph. The rebuild dialog keeps its glyph chip (no branded icon
exists for it).

app/sovran_systemsos_web/templates/index.html

@@ -239,7 +239,7 @@
   <div class="modal-overlay" id="update-modal" role="dialog" aria-modal="true" aria-labelledby="modal-title-text">
     <div class="modal-dialog">
       <div class="upd-header">
-        <span class="widget-chip chip-sovran upd-chip"><svg class="upd-chip-svg"><use href="#g-refresh"/></svg></span>
+        <img src="/static/icons/update.svg" class="upd-icon" alt="Sovran_SystemsOS Update" />
         <span class="upd-title-wrap">
           <span class="upd-title" id="modal-title-text">Sovran_SystemsOS Update</span>
           <span class="upd-sub">