fix(#1509): expose --nav-active-bg as a themeable token (#1571)

Red commit: 07a69e48ebc976caf4bf15f7a937e378a42ef718 (CI run: pending —
PR triggers first run)

Fixes #1509

## Problem

`--nav-active-bg` is defined in `public/style.css` (line 105) and used
by every
active-state nav link (`.nav-link.active`, `.nav-more-menu
.nav-link.active`,
plus the responsive blocks), but the customizer has never mapped it into
`THEME_CSS_MAP`. Result: presets, per-operator overrides, and
server-side
`theme.*` config can recolor every other nav token (`navBg`, `navBg2`,
`navText`,
`navTextMuted`) — but the active-pill background stays stuck on the
hardcoded
`rgba(74, 158, 255, 0.15)` (light) / dark-mode equivalent. Themes look
broken on
the one element users stare at.

## Fix

Triage-specified path, no scope creep:

- Add `navActiveBg: '--nav-active-bg'` to `THEME_CSS_MAP` in
`public/customize-v2.js`.
- Surface in the Theme tab's advanced color list (`THEME_COLOR_KEYS`
derives from
  the map; adding to `ADVANCED_KEYS` makes it render in the panel).
- Add label + hint so the input is self-explanatory.
- Seed defaults on the default preset's `theme` + `themeDark` so the
rendered
value matches today's hardcoded rgba and dark mode doesn't bleed the
light value.
- Document the new field in `config.example.json` per AGENTS.md config
rule.

## TDD

Red commit `07a69e48` adds `test-issue-1509-nav-active-bg.js` and wires
it
into the CI unit-test step. Assertions fail on master
(`THEME_CSS_MAP.navActiveBg`
is `undefined`; `applyCSS` does not write the variable). Green commit
`29d22ff5`
makes the assertions pass without touching any other test.

## Verification

- `node test-issue-1509-nav-active-bg.js` → 3/3 pass on this branch, 0/3
on master
- `node test-customizer-v2.js` → 59/60 (the 1 failure is pre-existing on
master,
  not caused by this PR — same failure with the diff stashed)
- pr-preflight: clean (all gates pass)

---------

Co-authored-by: corescope-bot <bot@corescope.local>
Co-authored-by: Kpa-clawbot <kpa-clawbot@users.noreply.github.com>
Co-authored-by: Kpa-clawbot <bot@meshcore-analyzer>

.github/workflows/deploy.yml

@@ -127,6 +127,8 @@ jobs:
           node test-issue-1420-tile-providers.js
           node test-issue-1438-marker-css-vars.js
           node test-issue-1562-observers-summary.js
+          node test-issue-1509-nav-active-bg.js
+          node test-issue-1509-detect-preset.js
           node test-live.js
           node test-issue-1532-live-fullscreen.js
           node test-xss-escape-sinks.js