Add in-editor deck selector for the Study-tab 'Add more cards' flow

[bootuz]

Summary

• The Study tab's "All caught up → Add more cards" button now opens the card editor directly (new no-arg cardForm route) instead of bouncing to the Library.
• In this picker mode the editor shows a Deck dropdown; the user must pick a deck before Save is enabled (no pre-selection). The chosen deck is preserved across saves for rapid entry and resets when you leave the editor.
• The Front field autofocuses on open and re-focuses after each save, so you can start typing immediately.
• Adding a card from a deck (the existing cardForm/{deckId} route) is unchanged: fixed deck, no selector. The brand-new-user "Go to Library" button is also unchanged.

Implementation

• CardFormViewModel: nullable deckId, pickDeck/decks/selectedDeckId state, onSelectDeck, canSave requires a deck, save() persists to the selected deck and preserves it on reset; checkNotNull(deckRepository) guard for picker mode.
• CardFormScreen: ExposedDropdownMenuBox deck selector (mirrors the existing ApkgImportScreen idiom) + FocusRequester autofocus.
• CardFormArgs.deckId nullable + Koin wiring; new cardForm nav route; StudyQueueScreen onAddCards callback.

Test Plan

• ./gradlew :app:testDebugUnitTest — green (3 new picker-mode unit tests + existing fixed-deck tests)
• ./gradlew :app:assembleDebug — debug APK builds
• ./gradlew :app:compileDebugAndroidTestKotlin — androidTest compiles (CardFormContentTest updated)
• Manual (emulator): Study tab → all caught up → Add more cards → editor opens with keyboard up + Front focused; Deck dropdown present; Save disabled until a deck is picked; after save the form clears, deck stays selected, Front re-focuses; back → reopening shows no deck pre-selected.

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