feat(assistant): wire the Nep casino guide on top of the Gemini SDK

[netqo]

Summary

• Replaces the Assistant placeholder with the cu-14 mockup port driven end-to-end by Google's Gemini SDK.
• domain/assistant exposes AssistantRepository + ChatTurn + Role so the feature layer never imports the SDK. data/assistant ships the Nep system prompt and the Gemini-backed implementation; di/AssistantModule provides the singleton GenerativeModel (pinned to gemini-2.5-flash) and binds the repository.
• feature/assistant is a sealed AssistantUiState (Welcome / Conversation) backed by a Hilt MVVM ViewModel that exposes a StateFlow. The Welcome body renders the hero + four canned suggestions; the Conversation body uses a LazyColumn with per-message UUIDs, a typing indicator, and a danger-accented bubble on failed turns. The input bar enforces the 300-char budget, sends on IME Send, and rides the soft keyboard via imePadding.
• Promotes PulsingDot out of LoginScreenEffects into ui/components so the Nep header and the login meta row share one implementation.
• Adds GEMINI_API_KEY to the Gradle script (read from gitignored local.properties into BuildConfig) and declares the INTERNET permission the SDK needs.

Persona + scope

NepSystemPrompt anchors the Neptunia-inspired voice (cheerful, casual, "~" suffix, refers to itself as Nep) and locks the scope to Stack Casino game rules / Provably Fair / wallet on Polygon. Off-topic asks get a polite refusal via the prompt itself instead of an extra UI state.

Notes

• GeminiAssistantRepository logs failures under the AssistantRepo tag with full stack traces and short-circuits with a clear message when the key is empty so the UI can render a useful bubble instead of a generic exception.
• LazyColumn keys are per-message UUIDs, not content-derived, so repeat questions inside the same minute never collide and crash the list.
• Error bubbles are excluded from the next-turn history so a failed call never poisons the model's context window.

Test plan

• ./gradlew assembleDebug
• ./gradlew ktlintCheck
• ./gradlew detekt
• ./gradlew testDebugUnitTest (AssistantViewModelTest: 8/8 passing)
• Installed on device, walked welcome -> suggestion -> reply, asked an off-topic question and saw the polite refusal, hit clear and confirmed the conversation resets.

Local setup

Add to stack/local.properties (gitignored):

GEMINI_API_KEY=<key from https://aistudio.google.com/apikey>

Then ./gradlew installDebug.