ui: simplify typography and semantic theme defaults [skip mobile-release]

[0xSero]

Summary

v2.0.1 Batch UI lane: conversation typography refinement plus semantic theme defaults. Two commits on top of main a8fb7bfb.

Transparency: implemented and runtime-gated by the DeepSeek-v4-pro lane; reviewed and approved by Fable-5. Computer-use visual recording sha256: 2c5c5bac670d1d87e2b78015f2d55faf21acda897c22f1d4ca10262967860a58 (evidence under /Users/sero/ai/projects/litter-release-evidence/v2.0.1/ui-gate/).

Commits

1. b18cd9c2 — chat: refine conversation typography [skip mobile-release] (6 files, +90/−23)
   • iOS: smaller monospaced code face (conversationCodePointSize), horizontally scrollable code blocks (fixedSize instead of max-width squeeze), tightened line/paragraph/block spacing, custom code-block renderer (language header + natural-width scroller).
   • Android: dedicated code typeface (Berkeley Mono → monospace fallback), code/code-block text sizing + line spacing (1.32), UI-test fixture text enriched with inline code + swift block.
2. 92a66e95 — appearance: make Studio the default theme [skip mobile-release] (4 files, +12/−8)
   • Default theme fallbacks codex-light → studio-light and chatgpt-dark → studio-dark on both platforms, via new defaultLightThemeSlug/defaultDarkThemeSlug (iOS) and DEFAULT_LIGHT_THEME_SLUG/DEFAULT_DARK_THEME_SLUG (Android) constants.
   • Font display-name renames ChatGPT (System) → System UI (3 hunks incl. iOS test); storageValue/raw value stays system; persisted preferences unchanged.

Supersedes

Supersedes #276 and #277. Do not auto-close — #276/#277 will be manually closed only after this batch merges.

Verification

• Targeted iOS unit tests (xcodebuild test -only-testing, iPhone 17 Pro simulator): 13 tests, 0 failures — LitterAppearanceModeTests (3), ThemeDefinitionTests (3), ConversationDisplayPreferenceTests (7).
• Fresh-install defaults verified headless: resolved palette = studio-light (accent #0d0d0d, bg #ffffff) + studio-dark (accent #ffffff, bg #181818), matching studio-light.json/studio-dark.json.
• Persisted-pref upgrade-install verified: stored selectedLightTheme=kitty-litter-light / selectedDarkTheme=kitty-litter-dark survive reinstall and are honored (kitty-litter palette resolved, not overwritten by Studio defaults).
• Mandatory sweep rg 'codex-light|chatgpt-dark' apps/: only legitimate theme asset/definition references remain; no fallback-default references.
• No submodule changes (codex 13595c36, ghostty a968e120 gitlinks unchanged).

Residual / follow-up

• Studio-light visual rendering is deferred to the G3 physical-device pass due to a pre-existing harness gap: ConversationDisplayUITestHarnessView does not wire syncSystemColorScheme, so the harness renders the dark palette regardless of system appearance. Studio-light resolution is proven via the app-group palette; Studio-dark is visually verified. Harness fix is tracked as a 2.0.2 follow-up (not in this PR).

[0xSero]

Superseded by #295 (v2.0.1 mega convergence). Equivalent mega commits: 98981fa8 (chat: refine conversation typography) + d49db8b4 (appearance: make Studio the default theme) — both patch-id and blob-verified. Closing; not merging.