fix(input): stop mouse from overriding keyboard selection in lists

[lukyrys]

Problem

During keyboard navigation in list views (file browser, download file list), the mouse interfered with the keyboard selection:

• A row scrolling under a stationary cursor fires mouseover/mouseenter without any real movement, hijacking the keyboard-selected row.
• The cursor only auto-hid after the idle delay, so it stayed visible during keyboard use.
• A redundant per-row Enter handler could activate the mouse-focused row instead of the keyboard-selected one.

Fix

A single input-mode flag (mouseActive) in the global mouse manager:

• keydown → mark mouse inactive + hide the cursor immediately
• real mousemove/mousedown → mark active + show cursor
• idle hide timeout → also mark inactive (cursor hidden ⟺ mouse inactive until the next real move)

Hover-driven selection (the manager's processHover and the per-component onmouseenter handlers in the file/download lists) now no-ops while the mouse is inactive, so a stationary cursor can no longer hijack the keyboard selection. Also drops the redundant per-row Enter handler in the file browser — the list area's central confirm handler already activates the selected row (removes an Enter dual-fire on the mouse-focused row).

Verification

• bun run check (frontend): 0 errors, 478 files
• Playwright against the dev server: hover-select, cursor-hide-on-keydown, hover-suppressed-during-keyboard, idle-timeout suppression, mouse re-activation, and Enter via the central handler all pass.