Design polish: signup screens use raw sp / type-size overrides + placeholder assets

[cestercian]

Minor design-token drift from M1b (non-blocking; merged in #14).

Raw sp / size overrides (CLAUDE.md says go through AppType, not raw sizes):

• feature/signup/WelcomeScreen.kt:104,110 — raw 16sp/12sp on the placeholder wordmark box
• feature/signup/WelcomeScreen.kt:122 — displayHero.copy(fontSize = 52.sp)
• feature/signup/DoneScreen.kt:91 — displayTitle.copy(fontSize = 40.sp)
• feature/signup/ProfileScreen.kt:120,160 — title.copy(44.sp), body.copy(18.sp)
• feature/signup/SignupChrome.kt:139 — raw 18sp

Fix options: add the missing large editorial sizes to the AppType ramp (Welcome/Profile use bigger headlines than the standard scale, matching iOS), and snap small labels to existing tokens (caption 12 / headline 18). Decide ramp-vs-snap per case.

Placeholder assets to replace (also from M1b, and shared with M0):

• Brand wordmark + Apple/Google/Mail glyphs on the auth screen (currently tinted letter boxes).
• Drop brand .ttf (Instrument Serif / Geist / Geist Mono) into res/font/ and swap the Type.kt fallbacks.
• Real launcher icon.
• A shared ScoreRing component (DoneScreen uses a one-off Canvas arc).

Low priority — do in a design-polish pass (part of M7) or when the brand assets land.