feat(theme): SVOS retheme — concept-round candidate

Re-skins every Booth surface in the SVOS design system (design-systems
palettes/svos @ ed2f8d8). Visual and interaction layer only: no route,
no copy, no ordering and no information-architecture change.

- _svos_tokens.css: SVOS semantic tokens vendored by copy, with the four
  [data-theme] scopes re-scoped onto prefers-color-scheme and
  prefers-contrast (dark, light, dark-hc, light-hc). Included into
  base.html's <style>; cached at startup like every other template.
- base.html: the accreted Australis sheet is rewritten against semantic
  tokens only. It also fixes four undefined variables (--line, --bg,
  --fg, --muted) that the keep/blur/reveal controls had been reading.
  The three SVOS devices each have exactly one job: reticle = selection
  (grid cursor, :target, picked option), hazard = irreversible (Wipe
  now, armed bulk delete), glow = live power (service dot, live bench).
- The flag list renders as wrapped chips, so a large flag set no longer
  pushes the grid below the fold. The list order is unchanged.
- IBM Plex Sans + JetBrains Mono load via Google Fonts with
  display=swap and system fallbacks (approved by booth-dev).
- view.html, doc.html: inline styles moved onto tokens.
- embed.js: fragment palette as custom properties scoped to .bk-ask;
  `.bk-ask-opt:has(input:checked)` still appears exactly once.
- Favicon (base.html + app.FAVICON_HREF, kept in sync): graphite tile
  with reticle corners.

Verified: 642 passed, the same count as the pre-change baseline.
Visual order matches document order on 32 renders (4 booths x 4 widths
x 2 schemes). A positive control, one tile given `order:-1`, is
detected by the same check.

booth/app.py

@@ -559,9 +559,10 @@ def _zip_filename(name: str) -> str:
 # the same icon. Keep the two in sync if the Booth's icon ever changes.
 FAVICON_HREF = (
     "data:image/svg+xml,%3Csvg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 32 32'"
-    "%3E%3Crect width='32' height='32' rx='7' fill='%23171a23'/%3E%3Ccircle cx='16' "
-    "cy='16' r='6' fill='none' stroke='%2342dcd1' stroke-width='2.5'/%3E%3Ccircle "
-    "cx='16' cy='16' r='2.2' fill='%2342dcd1'/%3E%3C/svg%3E"
+    "%3E%3Crect width='32' height='32' rx='7' fill='%2315191d'/%3E%3Cpath d='M7 12V7h5"
+    "M20 7h5v5M7 20v5h5M25 20v5h-5' fill='none' stroke='%23b2cd12' stroke-width='2.5' "
+    "stroke-linecap='round' stroke-linejoin='round'/%3E%3Ccircle cx='16' cy='16' r='3' "
+    "fill='%23b2cd12'/%3E%3C/svg%3E"
 )
 
 # The seam a verbatim report declares to get the Booth's chrome. ONE line, and