perf: workerize XTC conversion and add landscape clockwise flip

[varo6]

What this PR does

• Moves page conversion off the main thread into a worker pool, so long conversions do not block UI interactions.
• Adds sparse preview generation during conversion to reduce memory pressure and keep previews responsive on larger files.
• Adds an optional clockwise rotation for landscape pages and wires that option through UI + converter pipeline.

Why

• Improves converter responsiveness and throughput for multi-page inputs.
• Ensures landscape content can be exported with expected orientation when needed.

Fixes #10
Fixes #13