Replace PC-98 landing aesthetic with Heisei Twilight theme

Layered new heisei.css (loaded after pc98.css) over the public landing
page so the late-2000s/early-2010s anime cue lands subtly without a
ground-up rewrite. Internal VN/contracts/daemons screens are untouched.

What changed
- Twilight gradient backdrop (deep night → dusk plum → horizon rose +
  amber) replaces the chunky animated GIF background.
- Soft bokeh plates and a faint scanline overlay sit behind the content
  for atmosphere.
- Floating header is now frosted glass with a 1px hairline; the system
  status pills use Inter-tracked uppercase labels with desaturated
  cyan/magenta indicators.
- Vertical kanji masthead becomes a quiet Noto Serif JP column rather
  than an arcade box; English subtitle picks up a warm amber tone.
- Issue badge is a flat pill; LED heart still pulses but in muted rose.
- Empty hero state shows a faint tactical reticle (corner brackets) and
  bilingual JP/EN tagline — Ghost-in-the-Shell SAC HUD vibe.
- Mission and MAKIMA panels render as Heisei magazine columns with a
  Noto Serif JP headline, accent rule (amber for Mission, magenta for
  MAKIMA), and Inter body copy.
- Buttons are flat with hairline borders and a left-edge amber accent
  sweep on hover; the Login CTA keeps a warm gradient as the primary
  action.
- Bottom Velocity/Energy strip becomes a small HUD pill with corner
  bracket detailing.
- Mobile breakpoint tightens header, padding, headline size, and stacks
  the secondary CTAs back to a column.
- prefers-reduced-motion respected for the heart pulse and image pan.

How to verify
- cd frontend && npm install && npm run dev
- Compare http://localhost:5173 with the live soryu.co (PC-98 build)
- Toggle the Mission and MAKIMA panels; resize to 390x844 for mobile.

Implementation notes
- Single new file: frontend/src/styles/heisei.css (~16 KB).
- main.tsx imports heisei.css after pc98.css so the override wins.
- No JSX/component changes — design lives entirely in CSS, easy to
  revert by removing the import line.
- npm run build passes (vite v5.4, 91 kB CSS gzipped to 17.8 kB).

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