refactor: extract Android & iOS UI strings to resource files

[jvsena42]

Summary

Extracts all hardcoded user-facing UI copy out of Compose screens and SwiftUI views into proper string resource files, with consistent namespaced keys shared across both platforms.

Android — composeApp/src/androidMain/res/values/strings.xml

• ~190 namespaced keys (home_*, decks_*, deck_detail_*, deck_editor_*, edit_card_*, study_*, discover_*, paste_*, publish_*, onboarding_*, profile_*, friend_profile_*, settings_*, component_*, nav_*).
• 22 Compose files now use stringResource(R.string.…); interpolated text became positional format strings (e.g. %1$d of %2$d done · keep going!).
• EchoTab enum labels moved from String to @StringRes labelRes: Int, resolved at the composable call site.

iOS — iosApp/iosApp/Localizable.xcstrings (new String Catalog, 217 keys)

• 18 SwiftUI views reference the same keys as Android; format specifiers converted to C-style (%lld / %@).
• Plain literals use the key as LocalizedStringKey; parameterized strings use String(format: NSLocalizedString(...)).
• No project.pbxproj edits needed — the project uses Xcode 16 fileSystemSynchronizedGroups, so the catalog in iosApp/iosApp/ is auto-included.

Verification

• ✅ :composeApp:compileDebugKotlin passes.
• ✅ All 101 keys referenced in Swift exist in the catalog (cross-checked).
• ✅ SwiftLint passes with 0 warnings.

Notes / follow-ups

• 3 keys carry different wording per platform (kept native to each, since the files are separate): decks_empty_subtitle, deck_detail_delete_dialog_title, deck_detail_delete_dialog_message. Unify if desired.
• 5 iOS-only keys added (no Android equivalent yet): deck_editor_saving, deck_editor_title_placeholder_untitled, deck_editor_card_new, component_stats_bar_cards, common_coming_soon.
• Fixed a styling bug surfaced during extraction in DeckEditorView (ternary modifiers bound to only one branch → empty-card label was unstyled); now an if/else in a Group.
• Skipped intentionally: SF Symbol names, color hex, testTag/accessibilityIdentifier, #Preview mock data, log strings, pubky URIs, decorative emoji glyphs, dynamic variables.

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