fix(desktop): make the Run on chip idle at rest and show only the machine name #3444

Problem

The composer's Run on chip (apps/desktop/src/ui/Composer.tsx:1871-1881) always draws with bg-fill-subtle font-medium text-fg, so it looks pressed/active at rest, unlike the folder and Worktree chips beside it, which are transparent until hovered or on. It also reads "Run on This computer", the longest label in the strip.

Proposal

• Rest state like the folder chip: no fill, text-fg-muted, fill on hover and data-[state=open].
• Shorter label: the machine's icon plus its name only ("This Mac", "Studio"). Drop the "Run on" prefix from the visible text and keep it in aria-label and the tooltip. Use the device icon from the device-settings issue once it exists, falling back to Monitor.
• Give it the same container-width step as the folder chip (@max-lg/composer:max-w-[96px], icon-only under @max-sm).

UX: the strip reads [🖥 This Mac ▾] [📁 rustcode ▾] [⑂ Worktree], all three idle at rest; opening the menu fills the chip.

Acceptance criteria

• At rest the chip has no fill and matches the folder chip. It fills on hover and while open.
• The visible label is the machine name, with no "Run on" prefix. Screen readers still hear "Run on: ‹name›".
• Checked in the mock with ?machines=paired, with a screenshot in the PR.