Settings → Updates: scoped Reset + visible upgrade CTA + change diff link¶

Motivation¶

Three papercuts reported from the Settings panel:

Reset looked broken. The footer Reset button clears the browser-local preferences (localStorage["elb-prefs"]: theme, preview flags, telemetry / connection string). It worked, but it was shown on every section except Preview (showFooterActions = active !== "preview") — i.e. it appeared on Resources / Updates / AKS / … where there is nothing prefs-backed to reset (so it read as "the button does nothing"), and was hidden on the Preview section where it is most relevant.
No visible "update" button. Settings → Updates only rendered the "Manage upgrade → Open" link when the persisted state row already had a git_remote, and the actual start control lives on the separate /upgrade page. A new commit could be surfaced with no obvious way to act on it.
No way to see what changed. Neither surface showed which commits an update would bring in.

Reset is now scoped to the sections it affects. The footer Reset button appears only on Appearance, Preview, and Telemetry (the usePreferences / useTheme-backed sections). On the other sections it is hidden instead of appearing inert. Behaviour of the button itself is unchanged (still gated by the existing confirm dialog).
The self-upgrade link is always visible in Settings → Updates. When an update is available (a newer release tag, or a new commit when the preview channel is on) the row becomes a primary "Update now" call-to-action with an up-arrow icon; otherwise it is the neutral "Manage upgrade → Open" link. Both navigate to /upgrade.
"What's new" / "View changes" link. When an update is available and the remote is a GitHub repo, Settings → Updates shows a View changes link and the /upgrade "Start an upgrade" section shows View changes on GitHub — both open the GitHub compare/<running>...<latest> view so the operator can read the exact commit range.

No backend or IaC change. Reuses the existing upgradeApi typed client and the build-time __APP_COMMIT__ stamp.
Frontend only:
web/src/api/upgrade.ts — new pure helpers githubRepoBaseUrl() and githubCompareUrl() (normalise a GitHub HTTPS/SSH .git remote, strip credentials, and build the compare URL; return null for non-GitHub or an empty/equal range).
web/src/api/upgrade.test.ts — new unit tests for the helpers (13 cases).
web/src/components/SettingsPanel.tsx — PREF_BACKED_SECTIONS gate for the footer Reset; "What's new" row; always-visible "Update now / Manage upgrade" row.
web/src/pages/UpgradePage.tsx — "View changes on GitHub" link in the Start section.