Tighten mission/makima top space and use highlighted nav state

The CTA row is now a top navbar, so the old 'Close' label on the
active panel button is redundant — the button itself should look
like the current page. Two fixes here:

1. Replace 'Close' with a persistent active highlight.
   - LandingPage.tsx: button keeps its label ('Mission' / 'MAKIMA')
     and gains an is-active class + aria-current="page" when its
     panel is open. heisei.css renders the active state as a solid
     amber underline + amber text — same visual language as the
     hover indicator, just persistent.
   - The button still toggles the panel closed when clicked again,
     so the interaction is unchanged; only the labelling moved.

2. Remove the white-space band above mission content.
   - The legacy .modern-landing-page wrapper from pc98.css pinned
     itself at top: 120px and height: calc(100vh - 120px), matching
     the original 120px-tall floating header. The new header is
     72px (56px on mobile), which left ~50px of dead space below
     the navbar before the mission/makima content started.
   - Override the wrapper to top: 72px / height: calc(100vh - 72px)
     on desktop and 56px on mobile so it sits flush under the
     header.
   - Cover-content padding-top retuned for the new wrapper origin:
     desktop default 84px, mission-mode 72px (navbar 52 + 20 gap).
     Mobile default 76px, mission-mode 64px (navbar 48 + 16 gap).

Verified at 1280x720:
  navbar y=72-124 · mission headline starts y=144 · 20px gap ✓
  active Mission button: 'taisho-cta is-active', aria-current=page ✓

npm run build passes (CSS 92.6 kB / 18.1 kB gzip). Screenshots in
docs/heisei-screenshots/after-*.jpg refreshed against the tightened
layout.

6 files changed, 0 insertions, 0 deletions

diff --git a/docs/heisei-screenshots/after-01-desktop-default.jpg b/docs/heisei-screenshots/after-01-desktop-default.jpg
index 5e6a52b..4a35ffb 100644
--- a/docs/heisei-screenshots/after-01-desktop-default.jpg
+++ b/docs/heisei-screenshots/after-01-desktop-default.jpg
diff --git a/docs/heisei-screenshots/after-02-desktop-mission.jpg b/docs/heisei-screenshots/after-02-desktop-mission.jpg
index 213a961..6163d0e 100644
--- a/docs/heisei-screenshots/after-02-desktop-mission.jpg
+++ b/docs/heisei-screenshots/after-02-desktop-mission.jpg
diff --git a/docs/heisei-screenshots/after-03-desktop-makima.jpg b/docs/heisei-screenshots/after-03-desktop-makima.jpg
index 93049b3..c66521e 100644
--- a/docs/heisei-screenshots/after-03-desktop-makima.jpg
+++ b/docs/heisei-screenshots/after-03-desktop-makima.jpg
diff --git a/docs/heisei-screenshots/after-04-desktop-login-hover.jpg b/docs/heisei-screenshots/after-04-desktop-login-hover.jpg
index e510696..3c4b86f 100644
--- a/docs/heisei-screenshots/after-04-desktop-login-hover.jpg
+++ b/docs/heisei-screenshots/after-04-desktop-login-hover.jpg
diff --git a/docs/heisei-screenshots/after-05-mobile-default.jpg b/docs/heisei-screenshots/after-05-mobile-default.jpg
index 8b5760d..0ac8b14 100644
--- a/docs/heisei-screenshots/after-05-mobile-default.jpg
+++ b/docs/heisei-screenshots/after-05-mobile-default.jpg
diff --git a/docs/heisei-screenshots/after-06-mobile-mission.jpg b/docs/heisei-screenshots/after-06-mobile-mission.jpg
index 6b0ed8c..618c0c7 100644
--- a/docs/heisei-screenshots/after-06-mobile-mission.jpg
+++ b/docs/heisei-screenshots/after-06-mobile-mission.jpg