fix: Fall back to window duration for Claude and clear TUI buffer

[euxaristia]

Summary

Falls back to window duration (5h 0m) when Claude's session resets_at is null or unset, and explicitly clears the terminal buffer on TUI launch and redraws to prevent ghost text artifacts.

Changes

• Added format_duration in src/time.rs to format durations in seconds as countdown strings.
• Updated Bucket::into_window in src/parsers/claude.rs to use format_duration(seconds) when resets_at is null or missing.
• Updated src/tui.rs to call terminal.clear() on startup and on resize, and rendered Clear over the frame and panel areas to wipe dirty terminal cells.
• Updated Claude parser unit tests to assert duration fallback instead of Unknown.

Refs #30
Fixes #30

Test plan

• Ran cargo fmt --check (clean)
• Ran cargo clippy --all-targets --all-features -- -D warnings (clean)
• Ran cargo test (155 tests passing)
• Ran limits and verified the TUI launches cleanly with no terminal buffer bleed-through

[euxaristia]

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📥 Commits

Reviewing files that changed from the base of the PR and between 350d208 and b65a123.

📒 Files selected for processing (2)

• src/parsers/claude.rs
• src/time.rs

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