[PylotLight]

Follow-up to #656 as suggested: the reviewable slice (a), the compact-mode toggle off by default, rebased on current main with UI-only changes.

What this PR has:

• AppearanceSettings.CompactMode (pref_compact_mode, default false), LocalCompactMode CompositionLocal provided by S2AppTheme, Appearance switch plus strings on Android and iOS.
• Test pins: SettingKeysTest, SettingsScreenTest (new row off by default), StringKey coverage, iOS catalog pinned Android-only.

What it deliberately leaves out:

• No density readers yet, so nothing changes visually with the toggle off. Gated Home/Library/Settings/designsystem tweaks follow per screen with before/after screenshots against docs/design/design-language.md.
• No resume-hero size fix as a separate item: main's Home is now a JumpBackIn grid (the old ResumeHero card the branch patched no longer exists), so there is nothing to carry over. Say the word if a sizing issue remains on the new layout and I will fold it in.
• None of the old stack: no k8s/, JDK/minSdk/wrapper, scanner, theme consolidation or debug.keystore.

Still needed before merge (could not do here, no JDK in this environment):

• ./gradlew :android:app:testDebugUnitTest for SettingKeysTest, AndroidSettingsCatalogTest, StringKeyResourcesTest, SettingsScreenTest, SettingsIntegrationTest.
• Re-record the Roborazzi appearance goldens (docs/design/settings/appearance*.png). The new row changes the render.

Supersedes the (a) half of #656; the design-language pieces (flat indicators, placeholders, Settings tab, classic home/brand header, shelf play) stay as separate issues/PRs.

[timusus]

🤖 Claude here (an AI agent), reviewing for Tim.

Thanks for splitting this out so cleanly. We've decided not to add a compact-mode setting, so I'm closing this and #656.

• A setting instead of a decision. Our design guidelines treat a user toggle standing in for a design decision as something to avoid.
• Android already covers it. Display size and font size do this system-wide.
• It clashes with large-text support. Fixed, shrunk heights are what broke large text in Android: fixed-height chrome clips at 200% text (mini player, seek bar, library controls, queue) #730.
• Tablets adapt by width. Tablet layouts adjust to the window, not to a toggle.

If screens feel too sparse, we'd rather tighten the defaults for everyone; that's tracked in #863. If we ever do want a density option, it'll be built from the theme's sizing values rather than a CompositionLocal Boolean.

From #656, the Home shelf play button has landed, credited to you: a1cc974 (iOS follow-up in #865). The other design changes conflict with decisions made since:

• flat progress bars: design-language.md §3
• the Settings tab: Android: Settings pushes over the tab shell with Library still selected #791
• transparent settings rows: Android: Settings rows use trailing chevrons and one card per row #740
• the nav bar: §3
• artwork placeholders and the classic home: android: design review 1/5 — continuous-corner shape family, retire cookie shapes from controls #659 and Home redesign: smarter 'way back in' suggestions #633

Thanks for the push on this.