Document available icon names and clarify icon: usage

[thomasgallagher]

Clarify that icon: resolves to layered_ui/icon_NAME.svg and that an unknown name raises Propshaft::MissingAssetError. List the icons that ship with the gem (correcting the prior claim that only "home" did), and add an Available icons section to the dummy app showing the general-purpose set. Internal-UI chrome icons (mask shapes / baked theme colours) are noted as not usable via icon:.

[claude]

Review

Documentation-only PR that corrects and expands the icon reference: clarifies the behaviour, lists all icons that ship with the gem (previously just "home" was documented), distinguishes general-purpose from internal-chrome icons, and adds a visual Available icons section to the dummy app.

Verified: all 8 general-purpose icon names listed in HELPERS.md have corresponding icon_NAME.svg files in the gem. The dark:invert claim is accurate — .l-ui-navigation__item-icon carries that rule and .l-ui-icon does too, so the dummy app display will invert correctly. The internal-UI icons warned against (chevron_down, dark, etc.) are genuinely mask-based or carry baked colours, so the warning is correct.

No issues found.

✅ Ship