Add nightly build support

[wojtekn]

Related issues

• Related to STU-1759

How AI was used in this PR

Claude helped implement the changes. All code was reviewed and iterated on by the author.

Proposed Changes

• Enable automatic update polling for dev builds — previously dev builds had polling disabled entirely; manual "Check for Updates" already worked but nothing fired automatically
• 24-hour TTL for nightly update checks — timestamp stored in app.json (lastNightlyUpdateCheck) so the check fires at most once per day rather than on every launch
• buildUpdateFeedUrl({ channel? }) — extracted helper replacing the duplicated inline URL construction in setupUpdates and pollLinuxUpdates; adds optional channel param support
• switchToNightlyAndUpdate() — exported from updates.ts; points autoUpdater at ?channel=nightly feed and triggers an immediate check, letting Electron's existing update flow handle download and the restart prompt. On Linux, calls pollLinuxUpdates({ channel: 'nightly' }) directly
• maybePromptNightlySwitch() — detects signed-in Automatticians on stable builds via /rest/v1.2/read/teams, shows a one-time prompt offering to switch to nightly with a "Don't ask again" checkbox. On accept, calls switchToNightlyAndUpdate() directly — no manual download needed
• Hourly poller — startNightlyPromptPoller() re-runs the check every hour to catch users who sign in after launch

Testing Instructions

This PR requires sandboxing against 220097-ghe-Automattic/wpcom for the channel=nightly endpoint support.

To test with the sandbox:

1. Build a stable Studio build and sign in with an Automattician account
2. Point the update endpoint to the sandbox host
3. On launch, the nightly prompt dialog should appear
4. Accepting should trigger an immediate update to the latest nightly build via the normal Electron update flow (download in background → restart prompt)
5. Declining with "Don't ask again" checked should suppress the dialog on subsequent launches
6. Declining without "Don't ask again" should re-prompt on next launch

To test nightly auto-update polling:

1. Install a nightly (dev) build
2. Verify update check fires on launch if >24h since last check (lastNightlyUpdateCheck in ~/.studio/app.json)
3. Verify update check is skipped and scheduled for the remaining time if <24h since last check

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[wpmobilebot]

📊 Performance Test Results

Comparing 32d37b8 vs trunk

app-size

Metric	trunk	32d37b8	Diff	Change
App Size (Mac)	1340.07 MB	1340.07 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	32d37b8	Diff	Change
load	1770 ms	1726 ms	44 ms	⚪ 0.0%

site-startup

Metric	trunk	32d37b8	Diff	Change
siteCreation	9614 ms	9586 ms	28 ms	⚪ 0.0%
siteStartup	4926 ms	4912 ms	14 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)