Center mobile navbar buttons (kill leftover pc98 margin-top)

The bottom-CTA layout in pc98.css had a mobile-specific
margin-top: 20px on .taisho-cta, which carried over into the new
top navbar and pushed every button 20px below the navbar's
vertical center on mobile (visible misalignment at 390x844 — the
Mission/MAKIMA/Login pills were sitting near the bottom edge of
the bar).

- Add margin: 0 !important to .taisho-cta in heisei.css (and to
  the mobile-block override) so the legacy column-layout margin
  is wiped on both breakpoints.
- Switch buttons to height: 32px + line-height: 32px for stable
  vertical centering inside the 48px (mobile) / 52px (desktop)
  navbar.
- Force align-self: center on .cta-left/.cta-right so they don't
  inherit any stretched height from previously-applied rules.

Verified via Playwright DOM probe at 390x844:
  navbar y=56–104 (centerY 80) · buttons y=64–96 (centerY 80) ✓
And at 1280x720:
  navbar y=72–124 (centerY 98) · buttons y=83–113 (centerY 98) ✓

Screenshots in docs/heisei-screenshots/after-*.jpg refreshed.

7 files changed, 22 insertions, 6 deletions

diff --git a/docs/heisei-screenshots/after-01-desktop-default.jpg b/docs/heisei-screenshots/after-01-desktop-default.jpg
index 5db2c00..5e6a52b 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 014f9ff..213a961 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 8aff25f..93049b3 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 4ff0276..e510696 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 e7e3dab..8b5760d 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 6d174c7..6b0ed8c 100644
--- a/docs/heisei-screenshots/after-06-mobile-mission.jpg
+++ b/docs/heisei-screenshots/after-06-mobile-mission.jpg
diff --git a/frontend/src/styles/heisei.css b/frontend/src/styles/heisei.css
index 0245034..e12481d 100644
--- a/frontend/src/styles/heisei.css
+++ b/frontend/src/styles/heisei.css
@@ -413,6 +413,7 @@ body { background: var(--hz-night); color: var(--hz-ink); }
   border: 0 !important;
   outline: 0;
   padding: 8px 16px;
+  margin: 0 !important;             /* nuke pc98.css mobile-block margin-top */
   font-family: 'Inter', system-ui, sans-serif;
   font-weight: 500;
   font-size: 11px;
@@ -538,14 +539,29 @@ body { background: var(--hz-night); color: var(--hz-ink); }
   .cta-area {
     top: 56px;
     height: 48px;
-    padding: 6px 14px !important;
+    padding: 0 14px !important;     /* center via flex, not via padding */
     gap: 8px;
+    align-items: center !important;
   }
-  .cta-left { gap: 0; }
-  .taisho-cta { padding: 6px 10px; font-size: 10px; letter-spacing: 0.18em; }
-  .cta-right .taisho-cta { padding: 6px 14px; font-size: 10px; }
-  .cta-text { font-size: 10px; }
-  .cta-icon { font-size: 9px; }
+  .cta-left,
+  .cta-right {
+    height: auto !important;        /* don't inherit any column-layout height */
+    align-self: center !important;
+    align-items: center !important;
+  }
+  .cta-left { gap: 0; flex-direction: row !important; }
+  .taisho-cta {
+    padding: 0 10px;                /* let height drive vertical centering */
+    height: 32px;
+    line-height: 32px;
+    font-size: 10px;
+    letter-spacing: 0.18em;
+    align-self: center !important;
+    margin: 0 !important;           /* kill pc98.css mobile margin-top: 20px */
+  }
+  .cta-right .taisho-cta { padding: 0 14px; font-size: 10px; height: 32px; line-height: 32px; }
+  .cta-text { font-size: 10px; line-height: 32px; }
+  .cta-icon { font-size: 9px; line-height: 32px; }
 
   .modern-landing-page.manga-style .taisho-cover .cover-content {
     padding: 124px 20px 28px;