fix(web): redesign app update button

[incognitojam]

Background update downloads landed in #110, but the sidebar control still treated downloading as a call to action: it shared the isUpdateState styling branch with the states that want a click, so an automatic download lit the button and its changelog tooltip in update blue while needing no input at all. The downloaded state then offered a circular-arrow icon — the same metaphor this app uses for refresh and retry everywhere else, including the check-for-updates state of this very button.

This splits the tone decision into resolveDesktopUpdateButtonTone ("cta" | "quiet" | "idle"), so call-to-action colour is reserved for states that actually want a click: install, or retry a failed download. The intended flow is now visually staged:

1. Check for updates in the background (existing pollers, unchanged)
2. Update available → download starts automatically (feat(desktop): download updates in background #110, unchanged)
3. Downloading → muted progress ring around a download glyph, no CTA colour
4. Downloaded → CTA colour with an upward arrow, matching the update glyph in provider settings

	Downloading	Downloaded — ready to restart
Before
After

Full motion — indeterminate spin before the first progress event, gliding through the 10% progress broadcasts, then flipping to the CTA on completion:

ring.mp4

Implementation notes:

• The ring follows the ContextWindowMeter construct (track + dash-offset arc, -rotate-90 transform-gpu, motion-reduce:transition-none), with its stroke transitioned over 500ms so the 10%-step broadcasts from the main process glide instead of jumping.
• reduceDesktopUpdateStateOnDownloadStart reports downloadPercent: 0 until the first progress event, which would draw a zero-length arc, so 0 renders as an indeterminate spinning arc instead.
• A manual retry keeps the button natively disabled for the whole download; the quiet branch omits the disabled: dimming so background and manual downloads render identically.
• The changelog tooltip stays available while downloading, just without the update-tinted glass treatment.
• Tone is decided in desktopUpdate.logic.ts and covered by tests, so a regression to the always-CTA behaviour would now fail the suite.

Tests:

• vp test run apps/web/src/components/desktopUpdate.logic.test.ts apps/web/src/state/desktopUpdate.test.ts apps/web/src/components/desktopUpdate.toast.test.tsx apps/desktop/src/updates/DesktopUpdates.test.ts
• vp run --filter @t3tools/web typecheck
• Targeted lint for the changed TypeScript files

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Written by an agent (T3 Code, claude-fable-5).