Bar mode + ephemeral popup spawn (NON_UNIQUE)

[heath-toby]

Summary

Adds a persistent top-bar mode for WayTray, enabled by passing --bar to the client. In bar mode the window anchors to the top edge using layer-shell's Top layer with auto_exclusive_zone, runs without keyboard focus by default (picking it up on demand), skips the close-on-focus-loss logic that the popup uses, and ships its own dark CSS theme. Escape releases keyboard focus instead of closing.

Also flips the popup spawn model to gio::ApplicationFlags::NON_UNIQUE:

Previously, each Ctrl+Alt+Tab launch shared the same GApplication instance via D-Bus uniqueness. If the popup process ever ended up in a state where its layer-shell surface had been unmapped but app.active_window() still returned the destroyed window, subsequent present() calls became no-ops and the only fix was killall waytray. NON_UNIQUE makes every popup invocation its own short-lived process — no carry-over state, no stuck instance, and the existing focus-loss logic cleanly tears it down when dismissed. Bar mode keeps FLAGS_NONE so it remains a singleton.

Test plan

• Run waytray --bar — a top bar appears, anchored, with the styled background, never closes on focus loss.
• Press Escape on a focused bar — keyboard releases, bar stays visible.
• Run waytray (no flag) repeatedly — each invocation spawns its own process, opens a popup, closes cleanly on Escape or focus loss without leaving zombies.
• After dozens of popup open/close cycles, no killall waytray is needed.

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