feat(flatpak): add Flatpak packaging for universal distribution (#484)

* feat(flatpak): add Flatpak packaging for universal distribution

Adds a Flathub-ready Flatpak for the whisper.cpp engine, plus the minimal
runtime adaptations Flatpak actually needs — deliberately scoped down from the
earlier attempts (#416, #428), which bundled unrelated whisper.cpp GPU, audio,
and clipboard changes.

Packaging (packaging/flatpak/):
- app manifest, AppStream metainfo, and desktop entry (com.vocalinux.Vocalinux)
- generated offline Python dependency modules (flatpak-pip-generator)
- bundled shared modules: PortAudio, libayatana-appindicator, xdotool, xsel

Runtime changes (Flatpak-only, behaviour unchanged outside a sandbox):
- utils/paths.py: honour XDG_CONFIG_HOME/XDG_DATA_HOME so config, models and
  runtime data live under ~/.var/app/<app-id> in the sandbox
- autostart uses `flatpak run <app-id>` (the host launcher is always on PATH)
- text injection and keyboard backend detect a Wayland host reached via XWayland
  (no Wayland socket granted) and route through xdotool / pynput-on-X11
- tray indicator references the exported `<app-id>-*` host icon theme names

Also: a CI workflow building x86_64 + aarch64 bundles, and README / packaging
README / distro-compatibility doc updates.

Verified with a full flatpak-builder build + bundle install on aarch64:
pywhispercpp loads its native libraries without LD_LIBRARY_PATH, XDG paths
redirect correctly, bundled tools are present, and the entrypoint runs.
Manifest and builddir lint are clean and AppStream metadata validates.

Closes #167

* chore(flatpak): sync metainfo version to 0.14.0-beta

Also ignore flatpak-build-dir/ so local builder output stays untracked.

* refactor(flatpak): trim docs and comments after ponytail review

Drop redundant packaging README command blocks, shrink path/tray/autostart
comments, and stop ignoring a non-canonical build-dir name.

* fix(flatpak): close readiness gaps for v0.14.0-beta

Route log files through XDG data dirs so Flatpak keeps them under
~/.var/app, refresh AppStream screenshots to vocalinux.com, and use
the SPDX GPL-3.0-only license id.

* fix(flatpak): enable global hotkeys via evdev and --device=input

pynput over XWayland only receives keys for the focused X client. Prefer
evdev inside Flatpak and grant /dev/input so shortcuts work app-wide.

* fix(flatpak): inject text with ydotool/uinput on Wayland

xdotool only reaches XWayland clients, so typing appeared to succeed but
never hit native apps. Ship ydotool, grant --device=all for uinput, start
ydotoold on demand, and type via uinput inside the Flatpak.

* fix(flatpak): paste via wl-copy + Ctrl+V instead of typing

ydotool type injects character-by-character. Ship wl-clipboard, grant
--socket=wayland, and prefer clipboard paste for instant insertion.

* test(text-injection): expect ydotool default key-delay of 2

Matches the shipped VOCALINUX_YDOTOOL_KEY_DELAY default used when
clipboard paste falls back to character typing.

packaging/flatpak/python3-build-dependencies.yaml

@@ -0,0 +1,11 @@
+name: python3-build-dependencies
+buildsystem: simple
+build-commands:
+  - pip3 install --verbose --exists-action=i --no-index --find-links="file://${PWD}" --prefix=${FLATPAK_DEST} meson-python pyproject-metadata
+sources:
+  - type: file
+    url: https://files.pythonhosted.org/packages/16/7f/d1b0c65b267a1463d752b324f11d3470e30889daefc4b9ec83029bfa30b5/meson_python-0.19.0-py3-none-any.whl
+    sha256: 67b5906c37404396d23c195e12c8825506074460d4a2e7083266b845d14f0298
+  - type: file
+    url: https://files.pythonhosted.org/packages/1d/0b/da4851b1e2d9c40c9bd74c0abd94510a7d797da9ccde0a90e8953751ed4a/pyproject_metadata-0.11.0-py3-none-any.whl
+    sha256: 85bbecca8694e2c00f63b492c96921d6c228454057c88e7c352b2077fcaa4096