Show OpenRouter credit balance in settings

[GSmithBoltz]

What

Adds the user's OpenRouter credit balance to Settings, in the API Configuration section under the API key field: remaining balance with used/total detail, plus a manual refresh button. Loaded on screen open and on refresh.

Implementation notes

• New AccountRepository (domain) + AccountRepositoryImpl (data) fetching GET /api/v1/credits with the stored key via ktor, parsed with kotlinx.serialization; wired through Hilt.
• New dependency: ktor-client-core in :data (version managed by the existing openai-client BOM, same as the already-present ktor-client-okhttp).
• Graceful states: no API key set (hint text, no request), loading, and fetch failure (retry hint). Refresh is manual-only; typing in the key field does not trigger requests.
• UI uses only basics components, matching the existing settings rows.
• No migrations, no new permissions. The key never leaves the device except to OpenRouter itself.

Note: this branch shares the ktor-client-core catalog addition with #21 — if both are merged, whichever lands second will merge cleanly (identical change).

Testing

• :app:compileDebugKotlin, testDebugUnitTest, and ktlint for :domain/:data/:app pass (:ui has pre-existing violations on master; this change adds none).
• Manually tested on a physical device with a real key:

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