Trim helper text for navigating and add leaked shortcut and UI trimming

[soomtong]

We will improve some of the text in the screen and adjust the margin when the list navigation cursor moves to the bottom.

• Fix Ctrl+H leaking into fuzzy search textarea
• Add Ctrl+L navigation to grouped browse for previewing selected session
• Add Ctrl+P/N navigation to help screen for consistency with other modes
• Clarify summary cycle key label from [ / ] to [ or ] to avoid / key confusion
• Colorize [ or ] summary key in help screen
• Make session summary navigation wrap around (true cycle)
• Keep margin navigate to bottom edge

[subinium]

Thanks @soomtong — the margin-on-scroll and the leak fixes are keepers, and the wrap-around summary cycle matches what the help screen always implied. I pushed one fixup commit (d623f0c) to your branch so this can land as-is:

• Debug-build panic: the new selected - visible + 1 + margin can fire while selected < visible (viewport 10, margin 3 → triggers at row 7), which underflows usize — reproduced attempt to subtract with overflow in a debug build. Rewrote all three sites (browse / grouped / watch) as (selected + margin + 1).saturating_sub(visible); release behavior is identical.
• Overscroll clamp: grouped browse and agf watch lacked the max_offset clamp that adjust_scroll has, so the margin could leave up to 3 blank rows at the end of the list. Added the same clamp to both.
• Gates: clippy::collapsible_match on the Ctrl+L block (collapsed to Some((gi, Some(ci))), plus an early return to match the Esc handler), and cargo fmt over the new lines — these were the CI Format/Clippy failures.
• Added regression tests pinning the three scroll behaviors (no-underflow, margin honored, end clamp).

Will merge once CI is green.