feat: add compose variant history

[TimInTech]

Summary

• add a variant history to the Compose/Draft window
• keep up to 10 generated or edited variants
• add previous/next navigation and a variant counter
• keep manual in-place edits attached to the current variant

Scope

Included:

• Compose-window variant history state
• previous/next navigation
• i18n keys for variant controls
• tests for ring-buffer behavior, navigation, edits, reset behavior, and UI state

Excluded:

• no changes to app/blitztext_linux.py
• no speech-routing behavior
• no transcription/Whisper/AudioRecorder/hotkey rewrite
• no I-3 signature/name option
• no README, screenshot, or banner changes

Safety

• Thread lifecycle unchanged
• Voice-routing checkbox unchanged and still disabled
• Paste/clipboard behavior unchanged from I-1
• no real secrets or sk-... dummy values

Tests

• python3 -m compileall app tests
• QT_QPA_PLATFORM=offscreen WHISPER_GUI_TESTS=1 python -m pytest -q — 333 passed
• git diff --check
• scope check: only app/compose_window.py, app/i18n.py, tests/test_compose_window.py
• secret check: no real key, no sk-... dummy

Roadmap

Project #10 item Paket I: Compose-/Entwurfsfenster (Text-First) [Flaggschiff] remains In Progress.
I-3 stays out of scope.

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