fix(ui): adopt the canonical fleet theme and hue set C

[krisarmstrong]

Summary

ui/src/theme/msn-shared.css is now byte-identical to the canonical copy in
MustardSeedNetworks/.github (sha256 b073e5e15bab). The fleet
ci-conformance check 9 compares the two by hash and has been failing every
stem PR since .github#72 landed at 13:03:21Z — ci-conformance is in
ci-complete's needs: and CI Complete is required. stem's copy was
38eb15f3a815.

What the canonical file changes for stem: light --color-surface-border
#d9d2bd → #8e7f52 (1.45:1 → 3.48:1 on base, WCAG 1.4.11 wants 3:1 for a UI
edge), the whole dark palette re-derived at hue 43–51° as the night version of
the warm-cream light theme instead of cold steel, and --color-brand-gold
replaced by the shared --color-accent-gold (same value, one fleet decision).
stem's own darker success green was already the fleet's value.

The anchor moves to the fleet's hue set C (owner 2026-09-15): #1565c0 light,
#2d81e1 dark, in ui/src/theme/product-stem.css, with the theme-color meta
and the favicon following it. --color-brand-accent tracks the anchor because
it is both the hover fill behind every primary button (components/ui/Button.tsx)
and a text colour (Sidebar, ghost/outline variants): light #10509b carries
white at 7.95:1 and reads 7.60:1 as text on the page; dark #5a9fea reads
6.53:1 on the page ground and 5.81:1 on cards.

Three sites keep muted text while only the surface changes on hover
(SettingsDrawer.tsx:259,274, HelpDrawer.tsx:294). Under the canonical dark
palette that pair is #91896e on #312e25 = 3.88:1, down from 6.40:1
before this change, so they move to text-text-secondary (5.97:1). This is the
same finding run seed-20260916-1310 recorded on UI-SEED-7; UI-NIAC-9 and any
other adopter needs the same substitution, and the token itself is worth a
.github follow-up — the canonical file's comments measure text-muted against
base (5.17) and raised (4.61) but never against surface-hover.

Not changed here: the six module accents keep their current dark values — their
colour-blind-safe re-cut is UI-STEM-10's row, not this one.

Linked Issue

Fixes #1263

Type of Change

• Bug fix / chore (theme conformance; unblocks the repo's merge gate)

Risk

Low, and entirely visual. No Go, no API, no behaviour.

• Every dark surface, text and status value changes at once; the light theme
  changes two tokens. Both were reviewed against the canonical file's measured
  comments and re-measured here for the pairs stem actually renders.
• --color-text-accent takes the set C anchor as the row prescribes. Its dark
  value measures 4.09:1 on surface-raised, under AA — recorded rather than
  deviated from, and it has no consumer: rg 'text-text-accent' ui/src
  returns nothing (the token is defined in the @theme map only; the sidebar
  and buttons consume brand-accent).
• The six favicon rasters were regenerated from the SVG by
  scripts/build-favicons.js, which installs sharp transiently; nothing was
  added to package.json or the lockfile (git status clean there).
  android-chrome-512x512.png grew 14239 → 20786 bytes — encoder output, not
  content.

Testing Evidence

RED/GREEN on the gate that is blocking the repo, using .github main's own
script (the version every caller actually runs — the reusable workflow's SHA pin
does not freeze it, support-plan D-GH-3):

$ python3 ../.github/scripts/check-ci-conformance.py ~/Developer/.../stem   # main
::error::ui/src/theme/msn-shared.css has drifted from the canonical theme
    expected: sha256 b073e5e15bab (MustardSeedNetworks/.github ui/theme/msn-shared.css)

$ python3 ../.github/scripts/check-ci-conformance.py <this branch> | grep -c msn-shared
0

$ shasum -a 256 ui/src/theme/msn-shared.css
b073e5e15bab6366794986f671266f49ccf816a897b2b3be366e18b6ce6ca4a8

Gates (golangci-lint reports v2.13.2, the pinned CI version):

$ golangci-lint version
golangci-lint has version 2.13.2 built with go1.27.0 from 27774aa

$ make lint-go
0 issues.

$ make lint-frontend
Checked 255 files in 2s. No fixes applied.

$ make fmt-check
✓ All formatting checks passed

$ make test
[1/2] Backend (Go)   32 packages, all ok
[2/2] Frontend       62 files, 561 tests passed
EXIT=0

$ govulncheck ./...
No vulnerabilities found.
EXIT=0

$ scripts/check-token-discipline.sh
OK: blocking color-token discipline is clean.        exit=0
$ scripts/check-file-size.sh                         exit=0
$ scripts/check-filename-policy.sh                   exit=0
$ scripts/check-banned-vocabulary.py                 exit=0
$ npm run test:coverage && scripts/check-ui-coverage.sh
OK: UI coverage is at or above every floor.          exit=0

Storybook accessibility in real Chromium (color-contrast is blocking here,
preview.tsx test: 'error'):

$ npm run test:storybook      # after the SettingsDrawer/HelpDrawer fix
 Test Files  39 passed (39)
      Tests  111 passed (111)
EXIT=0

Note that this gate was already green with the 3.88:1 pair present: stories
render on the a11y addon's own #ffffff/#1a1a2e backgrounds, not on
surface-hover. The three sites were found by grepping the class pairs and
measuring them, not by the gate.

Past the gates to the artifact, because a gate can pass on a definition the
bundler drops:

$ make ui && rg -o 'accent-gold:[^;]*|--color-brand-primary:#[0-9a-f]*' internal/api/ui/assets/*.css
accent-gold:#8a6208}.dark{--color-surface-base:#181611
accent-gold:#b88a1e}:root{--color-brand-primary:#1565c0

Security and Release Checklist

• No new dependency, no lockfile change
• No secret, credential or endpoint touched
• No change to auth, CSRF, rate limiting or role gating
• govulncheck ./... clean
• No //nolint, no biome-ignore
• No customer-facing AI or banned vocabulary