fix(tray): show the tray icon on Linux by attaching its menu

[hiro-nikaitou]

The Windows tray refactor split the builder in two and the Linux branch kept only
the click handler, not the menu. tray-icon 0.21.3 (pinned in
src-tauri/Cargo.lock) documents two Linux facts that make that combination
invisible:

• TrayIconBuilder::menu / TrayIcon::new: "Linux: Sometimes the icon won't be
  visible unless a menu is set. Setting an empty Menu is enough."
• TrayIconEvent: "Linux: Unsupported. The event is not emmited even though the
  icon is shown"

So on Linux the indicator was built with no menu at all (popup_tray_menu could
never be reached), and hosts such as KDE Plasma never displayed it. With close
hidden to the tray there was then no way back to the window.

The Linux builder now attaches the same menu the other platforms attach; the
appindicator host opens it on click, and Open restores the window.

Verification, on the base revision ccc1eb8 and on the patched tree:

• a small script that extracts the cfg(not(any(target_os = "macos", target_os = "windows"))) builder and reports whether it attaches a menu:
  • base revision -> LINUX_MENU_ATTACHED=false
  • v0.6.2, the last release with a visible icon -> LINUX_MENU_ATTACHED=true
  • patched -> LINUX_MENU_ATTACHED=true
• cargo fmt --manifest-path src-tauri/Cargo.toml -- --check -> exit 0 (the CI
  format:check step); it parses tray.rs, so the edit is syntactically valid
• git diff --numstat -> 3 1 src-tauri/src/tray.rs

Not covered: nothing here compiles or runs the Linux target (libappindicator/GTK
are not available, and the branch is cfg-excluded on Windows), so the fix rests
on the crate's documented Linux behaviour plus the 0.6.2-to-0.6.9 regression
rather than on running the app on a Linux desktop.

Left alone: the Linux click handler and popup_tray_menu stay (they are dead on
Linux now) to keep this diff minimal; deleting them is an easy follow-up.

Fixes #316

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• src-tauri/src/tray.rs

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📝 Walkthrough

Walkthrough

The Linux tray builder now attaches the menu during tray icon creation. The Linux tray event handler remains unchanged.

Changes

Linux tray icon

Layer / File(s)	Summary
Attach the menu during tray icon creation src-tauri/src/tray.rs	The Linux tray builder passes the menu to the tray icon builder. The existing Linux tray event handler is unchanged.

Assessment against linked issues

Objective	Addressed	Explanation
Make the system tray icon appear on Linux [#316]	✅

Suggested reviewers: tonyantony300

Merge Risk: ⚪ Minimal · up to faf96

No actionable defect is established. The change is mergeable after normal checks, though confirming the tray flow on a Linux appindicator desktop would validate the intended behavior.

🚥 Pre-merge checks | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 1 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

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