Add Flatpak packaging for Linux app (#9)

Adds Flatpak manifest, .desktop file, and AppStream metadata so the
Linux app can be installed via Flatpak on any distro. Includes a build
script and updated README with all install options.

Your coworkers can test with:
```
./linux/build-flatpak.sh
flatpak run dev.zaptech.fuso
```

Author: arferreira

README.md

@@ -1,6 +1,6 @@
 # Fuso
 
-A lightweight macOS menu bar app for tracking your team's timezones. Native Swift, no Electron, no dependencies.
+A lightweight app for tracking your team's timezones. Native on macOS (Swift menu bar app) and Linux (GTK4 desktop app). No Electron, no dependencies.
 
 Fuso sits in your menu bar and shows the current time for everyone you work with. Optionally, it can track availability based on rotating schedules — useful for teams with members who have factory shifts, hospital rotations, or any recurring time blocks.
 
@@ -15,7 +15,7 @@ brew tap zaptech-dev/tap
 brew install --cask fuso
 ```
 
-### From source
+### From source (macOS)
 
 ```bash
 git clone https://github.com/zaptech-dev/fuso.git
@@ -24,6 +24,41 @@ cd fuso
 open /Applications/Fuso.app
 ```
 
+### Linux (Flatpak)
+
+```bash
+git clone https://github.com/zaptech-dev/fuso.git
+cd fuso
+./linux/build-flatpak.sh
+flatpak run dev.zaptech.fuso
+```
+
+Requires `flatpak`, `flatpak-builder`, and the GNOME SDK:
+
+```bash
+flatpak install flathub org.gnome.Platform//46 org.gnome.Sdk//46
+flatpak install flathub org.freedesktop.Sdk.Extension.rust-stable//24.08
+```
+
+### Linux (from source)
+
+```bash
+# Ubuntu/Debian: sudo apt install libgtk-4-dev
+# Fedora: sudo dnf install gtk4-devel
+# Arch: sudo pacman -S gtk4
+git clone https://github.com/zaptech-dev/fuso.git
+cd fuso/linux
+cargo build --release
+./target/release/fuso-linux
+```
+
+### CLI (all platforms)
+
+```bash
+cargo install fuso
+fuso
+```
+
 Requires macOS 13+ and Swift 5.9+.
 
 ## Configuration

linux/build-flatpak.sh

@@ -0,0 +1,18 @@
+#!/bin/bash
+set -e
+
+cd "$(dirname "$0")/.."
+
+# Generate cargo sources for offline build
+if ! command -v flatpak-cargo-generator &> /dev/null; then
+    echo "Installing flatpak-cargo-generator..."
+    pip install flatpak-cargo-generator 2>/dev/null || pip install --user flatpak-cargo-generator
+fi
+
+echo "Generating cargo sources..."
+flatpak-cargo-generator linux/Cargo.lock -o linux/cargo-sources.json
+
+echo "Building Flatpak..."
+flatpak-builder --force-clean --user --install build-dir linux/dev.zaptech.fuso.yml
+
+echo "Done. Run with: flatpak run dev.zaptech.fuso"

linux/data/dev.zaptech.fuso.desktop

@@ -0,0 +1,10 @@
+[Desktop Entry]
+Name=Fuso
+Comment=Track your team's timezones
+Exec=fuso-linux
+Icon=dev.zaptech.fuso
+Terminal=false
+Type=Application
+Categories=Utility;Clock;
+Keywords=timezone;clock;team;world;
+StartupNotify=true

linux/data/dev.zaptech.fuso.metainfo.xml

@@ -0,0 +1,29 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<component type="desktop-application">
+  <id>dev.zaptech.fuso</id>
+  <name>Fuso</name>
+  <summary>Track your team's timezones</summary>
+  <metadata_license>MIT</metadata_license>
+  <project_license>MIT</project_license>
+  <description>
+    <p>
+      Fuso is a lightweight desktop app for tracking your team's timezones.
+      It shows the current time for everyone you work with and can track
+      availability based on rotating schedules.
+    </p>
+  </description>
+  <url type="homepage">https://github.com/zaptech-dev/fuso</url>
+  <url type="bugtracker">https://github.com/zaptech-dev/fuso/issues</url>
+  <developer id="dev.zaptech">
+    <name>Zap Tech</name>
+  </developer>
+  <launchable type="desktop-id">dev.zaptech.fuso.desktop</launchable>
+  <content_rating type="oars-1.1" />
+  <releases>
+    <release version="0.1.0" date="2026-03-05">
+      <description>
+        <p>Initial release.</p>
+      </description>
+    </release>
+  </releases>
+</component>

linux/dev.zaptech.fuso.yml

@@ -0,0 +1,32 @@
+app-id: dev.zaptech.fuso
+runtime: org.gnome.Platform
+runtime-version: '46'
+sdk: org.gnome.Sdk
+sdk-extensions:
+  - org.freedesktop.Sdk.Extension.rust-stable
+command: fuso-linux
+
+finish-args:
+  - --share=ipc
+  - --socket=fallback-x11
+  - --socket=wayland
+  - --filesystem=~/.config/fuso:create
+
+build-options:
+  append-path: /usr/lib/sdk/rust-stable/bin
+  env:
+    CARGO_HOME: /run/build/fuso/cargo
+
+modules:
+  - name: fuso
+    buildsystem: simple
+    build-commands:
+      - cargo --offline fetch --manifest-path linux/Cargo.toml
+      - cargo --offline build --release --manifest-path linux/Cargo.toml
+      - install -Dm755 linux/target/release/fuso-linux /app/bin/fuso-linux
+      - install -Dm644 linux/data/dev.zaptech.fuso.desktop /app/share/applications/dev.zaptech.fuso.desktop
+      - install -Dm644 linux/data/dev.zaptech.fuso.metainfo.xml /app/share/metainfo/dev.zaptech.fuso.metainfo.xml
+    sources:
+      - type: dir
+        path: .
+      - cargo-sources.json