feat: native Linux (Ubuntu 24.04) port — GTK3 + Cairo, pure C

[ChanningYul]

Summary

Adds a native Linux port for Ubuntu 24.04 LTS in a new self-contained linux/ directory — zero changes to any Windows source. Like the original, it is written in pure C and reuses the project's portable assets (libs/miniaudio) and the resource/languages/*.ini translation files.

Implemented (core scope)

• ⏱ Transparent, borderless, always-on-top countdown window — Cairo/Pango text rendering with solid color or multi-stop gradient
• ✏️ Edit mode — drag to move, wheel to scale, Ctrl+wheel opacity, arrow keys to nudge; position/scale persist; click-through otherwise
• ⏲ Timer modes — clock / count-up / countdown, drift-free monotonic timing, 12/24-hour, seconds/milliseconds, 3 time formats (Default / 09:59 / 00:09:59)
• 🍅 Pomodoro — configurable interval sequence + loop count, per-session snapshot
• 📌 System tray (Ayatana AppIndicator) with full context menu
• 🔔 Desktop notifications (libnotify) + alarm audio (miniaudio: file playback or generated beep)
• 💻 Windows-compatible CLI — durations (25, 2h3m, 17 20t, 9 9 9t) and commands (s u p r pr q1-q3 v e h, p<n>), forwarded to the running instance over a Unix socket (single-instance)
• ⌨️ Global hotkeys via X11 XGrabKey, gracefully disabled on Wayland
• ⚙️ Config at ~/.config/catime/config.ini using the same section/key names as Windows — interoperable
• 🌍 i18n — runtime loading of resource/languages/*.ini (all 10 languages), English fallback, locale auto-detection

Verification (on real Ubuntu 24.04, GTK 3.24 / Cairo 1.18 / gcc 13.3)

• ✅ Clean build — zero warnings, zero errors
• ✅ Duration parser self-test: 13/13 (./catime --self-test)
• ✅ Drift-free countdown completion (3 s countdown fires exactly at 3 s)
• ✅ CLI forwarding to a running instance end-to-end
• ✅ Language loading: en and zh_CN (252 entries each)
• ✅ Desktop entry + icon install (desktop-file-validate clean)

Build & run

sudo apt install build-essential cmake pkg-config \
    libgtk-3-dev libcairo2-dev libpango1.0-dev \
    libayatana-appindicator3-dev libnotify-dev libx11-dev

cd linux && ./build.sh
./build/catime 25        # 25-minute countdown
./build/catime p         # Pomodoro

Full details in linux/README.md.

Notes / scope

• The Windows build is untouched — the Linux frontend lives entirely under linux/ with its own CMake project.
• Out of scope for this first iteration (present on Windows): tray icon animations & system monitor, full color/font/style picker dialogs, plugins, update checker, advanced text effects. The architecture (app/render/tray separation) is ready for them to be layered on.
• On Wayland, global hotkeys are unavailable (X11-only) and disabled automatically; everything else works.

[vladelaina]

@ChanningYul hi
Thank you very much for your work. However, I'm still undecided about whether to merge this for now. At the moment, I'm considering whether to implement it natively on each platform or use OpenGL to build a cross-platform solution instead. Since this decision affects the overall architecture of the project, I need some more time to think it through. I sincerely apologize for the inconvenience.