fix: actually delete Qwen3 cache when user removes the model

[mvanhorn]

Summary

• delete the on-disk Qwen3 cache when the user removes Qwen3-ASR 0.6B from Settings -> Models
• mirror how parakeetV3 is handled in the same switch: call Qwen3AsrModels.defaultCacheDirectory(variant: .int8) and FileManager.default.removeItem(at:) under the same #available(macOS 15, iOS 18, *) guard that modelIsCached uses

Why this fixes #69

removeCachedModelFiles(for:) in GhostPepper/Transcription/ModelManager.swift had this no-op for Qwen3:

case .qwen3AsrInt8:
    break // Qwen3 cache cleanup handled by FluidAudio internally

It does nothing. The companion modelIsCached(for:) (same file) DOES probe the actual cache directory:

case .qwen3AsrInt8:
    if #available(macOS 15, iOS 18, *) {
        return Qwen3AsrModels.modelsExist(at: Qwen3AsrModels.defaultCacheDirectory(variant: .int8))
    }
    return false

So:

1. User clicks remove -> deleteCachedModel(_:) -> removeCachedModelFiles(for:) -> Qwen3 case is no-op.
2. State briefly transitions to .idle, UI shows "Removing...".
3. Next refresh, modelIsCached returns true (cache files still on disk), so the model reappears as "Loaded" with no error.

The fix is a 4-line edit to actually delete the cache directory.

Test Plan

• xcodebuild ... build: passes.
• xcodebuild ... test: 190 unit tests pass on this branch.
• Pre-existing test failures on main (5 tests under GhostPepperTests/testAppStateArchives* and testFluidAudioRecordingUsesSpeakerFilteringSession) reproduce on upstream/main without this change. They look environment-related (Index out of range after XCTAssertEqual ("0") != ("1")) and are unrelated to model removal. Happy to look at those separately if useful.
• Manual: load Qwen3-ASR 0.6B, click Remove. The cache directory at Qwen3AsrModels.defaultCacheDirectory(variant: .int8) is deleted and the model row no longer reappears as "Loaded" on the next state refresh.

Closes #69