Web build and release CI: slidecraft-web, packaging, release workflow

- apps/slidecraft-web: SlideApp in the browser via eframe's web runner (trunk; WebGPU with
  WebGL2 fallback). Opens the sample deck (?blank for empty, ?show to present, ?webgl to force
  WebGL2). Open/Insert use the browser file picker through Services::open_async + inbox;
  Save/Export download; dropped files are read asynchronously.
- ui-egui: native-only synchronous drop handling (wasm uses the inbox), real clock on wasm
  (js_sys::Date::now), About shows build commit/date when SLIDECRAFT_BUILD_SHA/DATE are set.
- .github/workflows/release.yml (push to `release` / workflow_dispatch): macOS universal
  signed+notarized dmg, Windows x64/x86 signed msi + portable zip, Linux AppImage/deb/rpm/tar.gz
  (x86_64, aarch64), Flatpak bundle, FreeBSD tar.gz (vmactions), web zip; draft GitHub Release
  "SlideCraft v<version>" from [workspace.package] version. Same secret names as DesignCraft.
- freebsd.yml and packaging-lint.yml CI, ported from DesignCraft.
- packaging/: macOS (Info.plist, entitlements, cert import), Windows (WiX with new UpgradeCode,
  .slidecraft ProgId, Open-with for pptx), Linux (.desktop, AppStream, MIME, nfpm, Flatpak),
  FreeBSD and web scripts; shared setup in packaging/common.sh.
- apps/slidecraft/build.rs embeds icon + VERSIONINFO on Windows; slidecraft.icns/.ico generated
  from the committed PNGs by packaging/icons.sh.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -9,3 +9,6 @@ plan/
 /.claude/
 /craft-fonts/
 packaging/env.sh
+/packaging/linux/flatpak/cargo-sources.json
+/build-flatpak/
+/.flatpak-builder/