updater 2/5: bump electron-updater to 6.8.9

[insomnious]

First of a stack of five (this one, then resolver, status push, UX, docs). Together they rework auto-updating end to end. What the stack fixes and adds:

• Beta users were offered older stables as "updates" because releases were picked by publish date, not version. Fixed with our own resolver.
• Stable updates sat dark until a human cleared the prerelease flag on GitHub. CI sets it now.
• The renderer polled for update status once, 15s after launch, and missed anything slower. Status is pushed live, with a 4-hourly re-check for long sessions.
• Differential updates: a patch downloads roughly 10% of the 360MB installer instead of all of it.
• Downgrades only happen on purpose, offered explicitly when switching to Stable from a newer build, never disguised as an update.
• Users can see what's happening: live download progress, error notifications with a working retry, a visible installer instead of a silent one, and "what's new" after updating.
• The whole cycle is testable offline with a mock feed and local builds, and the updater is a logged state machine with 84 tests.

This PR is the foundation. It brings electron-updater from 4.6.5 (2021) to 6.8.9. We tried taking electron-builder to 26 at the same time, but its new pnpm module collector mangles nested dependency versions and shipped us a broken asar, so builder stays on 24.13.3 until that's fixed upstream. scripts/verify-packaged-asar.mjs now runs in CI so that class of breakage can't ship silently again. Also drops the unused electron-updater dep from the renderer.

Testing completed on live GitHub

#	do this	expect in the UI	expect in the log
1	Launch 9.0.0, Stable	nothing, then "Vortex will update on restart" [Restart Now]	checking → downloading 9.0.1 (patch), Differential download, Verifying signature, staged 9.0.1 (patch). 9.0.1 picked over the newer-dated beta.
2	Quit Vortex, cancel the UAC prompt	nothing installs	Installing staged update on quit, elevate.exe. Next launch re-stages.
3	Restart Now, accept UAC	NSIS wizard, then "Vortex was updated to 9.0.1" [View changes]	Vortex Version "9.0.1", checking → idle
4	Switch channel to Beta	"Vortex 9.1.0-beta.1 is available to download" [What's New, Download]	checking(manual) → available 9.1.0-beta.1
5	What's New	dialog shows the scratch release body
6	Download	"Downloading Vortex 9.1.0-beta.1 (n%)", no buttons, then "ready to install" [What's New, Restart Now]	downloading … (update), staged … (update)
7	Restart Now, accept UAC	9.1.0-beta.1 boots with "was updated"	Vortex Version "9.1.0-beta.1", checking → idle
8	Switch channel to Stable	"Vortex 9.0.1 is a downgrade and older than your current version" [More]	checking(manual) → downgrade-offered 9.0.1
9	More, Stay on current version	notification gone; Check now says up to date	Downgrade offer declined, → idle
10	Beta, then Stable again	offer comes back	→ downgrade-offered 9.0.1
11	More, Downgrade	"Downgrading to Vortex 9.0.1 (n%)", then "Vortex will update on restart" [Restart Now]	Downgrade download confirmed, downloading … (downgrade), staged … (downgrade)
12	Restart Now, accept UAC	9.0.1 boots. No "Downgrade detected" dialog.	Expected downgrade detected, skipping warning {"from":"9.1.0-beta.1","to":"9.0.1"}

Screenshots