fix: restore keys when loading generation history

[laceyp99]

Summary

• restore normalized and enharmonic Core key spellings to one of the 12 visible UI choices when loading generation history
• preserve the current key and warn without blocking the remaining history load when saved metadata contains an invalid key
• make the history sidebar and generation cards follow Gradio's light and dark theme colors

Fixes #26

Testing

• uv run --locked --extra dev ruff format --check . - passed
• uv run --locked --extra dev ruff check . - passed
• uv run --locked --extra dev pytest -q - passed (45 passed, 1 platform-specific skip)
• uv build - passed
• manually loaded generated black-key and natural-key history entries - passed
• visually checked the history sidebar in light and dark modes - passed

[laceyp99]

Update summary

• Addressed the single code-review finding (RC-001): test_history_uses_theme_aware_styles asserted literal CSS substrings inside APP_CSS, coupling the test to implementation strings rather than behavior.
• Dropped those substring assertions and renamed the test to test_render_history_html_uses_theme_aware_classes, keeping only the render-level class="history-empty" check. A CSS refactor now won't break the test without a user-visible change.
• No production code changed in this update.

Testing

• uv run --locked --extra dev ruff format --check . - passed
• uv run --locked --extra dev ruff check . - passed
• uv run --locked --extra dev pytest -q - passed (45 passed, 1 platform-specific skip)
• uv build - passed

[laceyp99]

Looks good to merge! This additional commit makes the tests decoupled from the potentially shifting CSS substring values.