Cherry-pick update indicator styling: remove post-install button and Ready hint

[Copilot]

Cherry-picks just the button/animation/hover styling from eli/then-dormouse without the test commands infrastructure, and makes the following cleanup changes:

What changed:

• Removed the "Update Complete" post-install title bar button — the showPostInstall/dismissPostInstall methods and all related state (postInstallTimer, postInstallOptions, pendingPostInstall) have been removed from UpdateTitleBarEntry and UpdateTitleBarContribution. The button that previously appeared and faded after a version update no longer shows.

• Removed the "Installed. Restart?" hint from the Ready state — when an update is ready to apply (Ready state), the title bar button now just shows "Update" with the prominent background. The sliding hint text "Installed. Restart?" and the periodic flash animation have been removed.

• Removed testShowPostInstall method — this was a dev-only helper used by the test commands infrastructure that is not being carried over.

• Removed update.test.contribution.ts and restored its .gitignore entry — the test commands file that was added to the repo in eli/then-dormouse is not included in this PR.

• Removed post-install CSS classes from updateTitleBarEntry.css — .post-install and .post-install-fade styles cleaned up.

What's preserved:

• All button/animation/hover styling (shimmer progress underline, percent fill underline, slide-out hint animation, prominent button style for actionable states)
• The detectVersionChange flow still calls renderPostInstall() on the tooltip and auto-shows the tooltip on version upgrade — only the title bar button label is removed
• Hints still appear for AvailableForDownload and Downloaded states