refactor: consolidate assets/icon into assets/icons

[StudentWeis]

Motivation

Currently SVG icons are split between two directories that differ only by pluralization:

• assets/icon/ — 14 UI icons referenced directly by source code (e.g. icon/check.svg, icon/settings.svg).
• assets/icons/ — 3 icons consumed indirectly by gpui-component's Notification at runtime (allowlisted in scripts/check/icon_allowlist.txt).

Having both icon/ and icons/ is confusing, inconsistent, and hurts discoverability. There is no technical reason to keep them separate.

Proposed Solution

• Move all 14 SVGs from assets/icon/ into assets/icons/.
• Update the 18 references across 5 Rust source files (src/gui/board/*.rs, src/gui/panel/*.rs) from icon/xxx.svg to icons/xxx.svg.
• Delete the now-empty assets/icon/ directory.
• No behavioral change; scripts/check/check_icons.py and scripts/precheck.sh must still pass.

Acceptance Criteria

• assets/icon/ no longer exists.
• All icons live under assets/icons/.
• No icon/ path references remain in Rust sources.
• scripts/precheck.sh passes.

Scope

gui