Add Flatpak and Flathub packaging support

Add all files needed to package Yaak as a Flatpak for distribution on
Flathub. Uses the deb-extraction approach recommended by Tauri docs:
the manifest downloads the existing .deb release artifact, extracts it,
and installs the binary/resources into the Flatpak /app prefix.

Includes:
- AppStream metainfo.xml with app metadata and branding
- Flatpak manifest (app.yaak.desktop.yml) for x86_64 and aarch64
- Properly sized icons (128/256/512px) named per Flatpak conventions
- Helper script to update manifest hashes for new releases

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flatpak/app.yaak.desktop.yml

@@ -0,0 +1,79 @@
+# Flatpak manifest for Yaak
+#
+# To build and test locally:
+#   flatpak install flathub org.gnome.Platform//47 org.gnome.Sdk//47
+#   flatpak-builder --force-clean --user --install-deps-from=flathub --repo=flatpak-repo build flatpak/app.yaak.desktop.yml
+#   flatpak --user remote-add --if-not-exists yaak-local flatpak-repo
+#   flatpak --user install yaak-local app.yaak.desktop
+#   flatpak run app.yaak.desktop
+
+id: app.yaak.desktop
+runtime: org.gnome.Platform
+runtime-version: "47"
+sdk: org.gnome.Sdk
+command: yaak-app
+
+finish-args:
+  # Display (Wayland + X11 fallback)
+  - --socket=wayland
+  - --socket=fallback-x11
+  - --share=ipc
+  # GPU rendering (WebKitGTK)
+  - --device=dri
+  # Network access (required — Yaak is an API client)
+  - --share=network
+  # File access for importing/exporting and Git project directories
+  - --filesystem=home
+
+modules:
+  - name: yaak
+    buildsystem: simple
+    build-commands:
+      # Extract the deb package
+      - ar -x yaak.deb
+      - tar -xf data.tar.gz
+
+      # Install the binary
+      - install -Dm755 usr/bin/yaak-app /app/bin/yaak-app
+
+      # Install bundled resources (plugins, node runtime, protoc, etc.)
+      - cp -r usr/lib/yaak /app/lib/yaak
+
+      # Install desktop file, updating Exec and Icon to match Flatpak conventions
+      - install -Dm644 usr/share/applications/yaak.desktop /app/share/applications/app.yaak.desktop.desktop
+      - desktop-file-edit --set-key=Exec --set-value=yaak-app /app/share/applications/app.yaak.desktop.desktop
+      - desktop-file-edit --set-icon=app.yaak.desktop /app/share/applications/app.yaak.desktop.desktop
+      - desktop-file-edit --set-key=StartupWMClass --set-value=yaak-app /app/share/applications/app.yaak.desktop.desktop
+
+      # Install icons from deb (small sizes) and from this repo (large sizes)
+      - install -Dm644 usr/share/icons/hicolor/32x32/apps/yaak-app.png /app/share/icons/hicolor/32x32/apps/app.yaak.desktop.png
+      - install -Dm644 usr/share/icons/hicolor/128x128/apps/yaak-app.png /app/share/icons/hicolor/128x128/apps/app.yaak.desktop.png
+      - install -Dm644 icons/256x256/app.yaak.desktop.png /app/share/icons/hicolor/256x256/apps/app.yaak.desktop.png
+      - install -Dm644 icons/512x512/app.yaak.desktop.png /app/share/icons/hicolor/512x512/apps/app.yaak.desktop.png
+
+      # Install AppStream metainfo
+      - install -Dm644 app.yaak.desktop.metainfo.xml /app/share/metainfo/app.yaak.desktop.metainfo.xml
+
+    sources:
+      # The deb package from GitHub releases
+      # Update the URL and sha256 for each release
+      - type: file
+        dest-filename: yaak.deb
+        url: https://github.com/mountain-loop/yaak/releases/download/v2026.2.0-beta.10/yaak_2026.2.0-beta.10_amd64.deb
+        sha256: "FILL_IN_SHA256_HASH"
+        only-arches:
+          - x86_64
+      - type: file
+        dest-filename: yaak.deb
+        url: https://github.com/mountain-loop/yaak/releases/download/v2026.2.0-beta.10/yaak_2026.2.0-beta.10_arm64.deb
+        sha256: "FILL_IN_SHA256_HASH"
+        only-arches:
+          - aarch64
+
+      # Icons (from this repo)
+      - type: dir
+        path: icons
+
+      # AppStream metainfo (from this repo)
+      - type: file
+        path: app.yaak.desktop.metainfo.xml