Improve result pane scroll responsiveness

[BeLeap]

Motivation

• The result-pane scrolling with j/k felt laggy because the UI only processed one input event per 100ms tick and used a bounded action channel that could become a bottleneck for repeated keypresses.

Description

• Switch the UI action channel from a bounded channel to an unbounded channel in src/ui/manager.rs to avoid blocking on repeated keypresses.
• Increase the UI tick rate from 100ms to 16ms in src/ui/manager.rs to reduce perceived input latency.
• Drain all pending crossterm events each frame by replacing the single poll check with a while poll(...) loop in src/ui/manager.rs so held/repeated keys are handled promptly.
• Add a journal entry at journal/2026-03-18/fix_scroll_lag.md documenting the investigation, root cause, and fix approach.

Testing

• Ran cargo test, which completed successfully (no failing tests).
• Ran cargo check, which completed successfully (build passed checks).

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Codex Task