Update now: download, verify and install a release in one click (#1118)

Where SolidCraft can replace itself safely, the Update window gets Update now:
- AppImage: the new AppImage is downloaded next to the old one, verified and renamed over
  it; Restart now starts it as SolidCraft exits.
- Windows MSI: the verified .msi starts (msiexec /i … /passive) once SolidCraft has closed,
  after the usual questions about unsaved designs.
- Windows portable zip / standalone exe: the running solidcraft.exe is renamed to .old, the
  new one takes its name (solidcraft-cli.exe too), Restart now; .old goes at the next start.
- macOS: the verified .dmg is opened (the app is ad-hoc signed and not notarized, so it isn't
  replaced behind the user's back).
- Flatpak, Snap, deb, rpm, tar.gz: never replaced; the window shows the command.

Every package's SHA-256 must match the release's SHA256SUMS.txt (HTTPS, same release); a
mismatch deletes the download and changes nothing. With PUBLIC_KEY set in update/install.rs,
SHA256SUMS.txt.minisig must also verify (minisign-verify, MIT) with a trusted comment naming
the release. release.yml signs the list when the UPDATE_SIGNING_KEY secret exists and fails
when the app expects signatures but the secret is missing; docs/releasing.md has the steps.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -2019,6 +2019,12 @@ dependencies = [
  "autocfg",
 ]
 
+[[package]]
+name = "minisign-verify"
+version = "0.3.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "871285dc19d8d0ebe0eef3d0e99a205f2a71363b122632cbbfa4a6c370a960ce"
+
 [[package]]
 name = "miniz_oxide"
 version = "0.8.9"
@@ -3593,14 +3599,17 @@ dependencies = [
  "egui-wgpu",
  "image",
  "log",
+ "minisign-verify",
  "pollster 0.4.0",
  "semver",
  "serde",
  "serde_json",
+ "sha2",
  "solidcraft-engine",
  "solidcraft-gpu",
  "ttf-parser",
  "web-time",
+ "zip",
 ]
 
 [[package]]