refactor: native-first UI sweep (Material 3 Expressive + SwiftUI)

[jvsena42]

What & why

Sweeps the remaining bespoke custom chrome across the app onto native platform components, per the CLAUDE.md "Native-first UI" rule (design/DESIGN_GUIDELINE.md §4). Brand tokens (accent, type, radii) are applied to native components rather than rebuilding chrome from Box/Row/Capsule primitives. Continues the pattern established by the EchoTabBar → ShortNavigationBar change.

Android (Compose / Material 3 Expressive)

• Echo buttons rewrapped on native M3 (same public API): EchoPrimaryButton→Button, EchoSecondaryButton→FilledTonalButton, TagChip→InputChip.
• Headers → TopAppBar + IconButton/FilledIconButton: EditCard, DeckDetail, Settings, Profile, Paste, DeckEditor.
• BasicTextField → OutlinedTextField; custom pills/badges → AssistChip.
• Progress bars → LinearProgressIndicator (Home, Study); divider → HorizontalDivider; Profile error banner → Snackbar; inline CTAs → native Button/OutlinedButton; SRS grade buttons rebuilt on Button; onboarding link → TextButton.
• Kept brand-custom: study card flip, DeckTile/AuthorRow/CardPreviewRow/StatsBar, hero/cover/avatar layouts.

iOS (SwiftUI)

• New EchoButtonStyles.swift — reusable ButtonStyles mirroring the Android Echo buttons, replacing repeated Capsule().fill() chrome across 9 views.
• Custom progress bar → ProgressView. Native TabView/.sheet/Divider already in place; tag chips/hero cards kept brand-custom (no native iOS chip).

Note: the iOS app is drafted/inert (Koin bootstrap still commented out), so it is SwiftLint-verified only — no end-to-end Xcode build runs yet. The Xcode project uses PBXFileSystemSynchronizedRootGroup, so the new file auto-compiles in-target.

Cross-cutting

• Shadow color literals extracted to tokens (shadowAccent, shadowElevation{Low,Medium,High,XHigh}) in both EchoColors.kt and EchoColor.swift; 0 inline shadow literals remain.
• CLAUDE.md: added rule to always import symbols rather than reference them fully-qualified inline (Kotlin + Swift).

Verification

• ./gradlew detektAll :composeApp:assembleDebug → BUILD SUCCESSFUL
• ./gradlew lintSwift → 0 violations

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