feat: surface app version and update awareness

[fuzzzerd]

Summary

Adds a Help → About dialog showing the host version and a row per loaded plugin. Plugins that opt in via a new capability interface get a per-row "Check for updates" affordance; the host has its own check against the public releases endpoint. The host never hardcodes a plugin's update channel.

Design

A new capability interface in SharpFM.Plugin:

public interface IUpdateCheckable
{
    Task<UpdateCheckResult> CheckForUpdatesAsync(CancellationToken ct);
}

public sealed record UpdateCheckResult(
    bool UpdateAvailable,
    string? LatestVersion,
    Uri? ReleaseUrl,
    string? Notes);

Opt-in, not a member of IPlugin:

• Bundled plugins ship with the host binary and have no independent update channel — forcing them to implement an empty check would be ceremony.
• IPlugin.Version already covers "what am I running?" — every plugin reports it. Check-for-updates is a separate concern.
• The pattern matches the existing IPanelPlugin : IPlugin opt-in extension in SharpFM.Plugin.UI.
• No breaking change to existing plugins.
• HttpClient / transport concerns stay out of the headless plugin contract.

Each plugin decides where its updates live. The host iterates [host, ...plugins] uniformly and runs CheckForUpdatesAsync on entries that opted in. The host has no compile-time knowledge of any specific plugin's release source.

Host self-check

HostUpdateCheck implements IUpdateCheckable against api.github.com/repos/fuzzzerd/SharpFM/releases/latest:

• Anonymous (no auth header) — anonymous requests are rate-limited to 60/hour per IP, fine for explicit user action.
• The /releases/latest endpoint already excludes pre-release tags, so beta-channel filtering is the endpoint's responsibility, not ours.
• Strips v prefix from tags and +commit-hash build metadata from the running AssemblyInformationalVersion before comparing.
• Catches every exception except OperationCanceledException — fail silently if GitHub is unreachable.
• Adds a User-Agent header (GitHub rejects requests without one).

About dialog UX

• Title + host version at top.
• "Check for updates" / "View release" / "GitHub" buttons for the host. "View release" appears only after a successful check that found an update.
• Plugin list below: each row shows name + version. Plugins that opted in get a "Check for updates" button; plugins that didn't, don't.
• Status text appears inline per row after the check resolves ("Update available: 2.0.0", "Up to date.", or "Could not check for updates.").
• Dialog is [ExcludeFromCodeCoverage] (Avalonia plumbing); the view models behind it are fully unit-tested.

Tests

• Plugin contract (6 new) — interface shape, capability is standalone (not IPlugin-derived), record equality.
• HostUpdateCheck (11 new) — fake HttpMessageHandler for newer/equal/older versions, 404, network exception, malformed JSON, cancellation propagation, v-prefix and +commit stripping, release-notes capture.
• AboutEntryViewModel (7 new) — CanCheckForUpdates gating, status transitions, fail-silent on exceptions, INPC on Status, cancellation propagation.
• AboutViewModel (4 new) — host always checkable, plugins checkable iff they implement IUpdateCheckable, homepage URL exposed.

All 1454 tests pass (+28 new).

Test plan

• Manually launch the app and open Help → About; confirm host version renders and plugin rows appear.
• Click host "Check for updates" with network available — confirm result text appears.
• Click "GitHub" — confirm the public repo opens in the system browser.

[github-actions]

Test Results

✔️ Tests 1454 / 1454 - passed in 11.3s
✔️ Coverage 79.51% - passed with 70% threshold
📏 15390 / 17814 lines covered 🌿 5223 / 8110 branches covered
🔍 click here for more details

✏️ updated for commit 6a41645