feat(client): minimise to a system tray indicator (GNOME)

A dedicated monochrome button in the nav, immediately left of the notification
bell, hides the window to a tray indicator. Linux only for now; Windows is
being done on that OS, and the capability is declared per platform so the
button never appears where the desktop shows no indicator -- there it would
hide the window for good.

Hides, never closes: `window-all-closed` quits the app, so closing here would
make "minimise" mean "exit" and drop the session, the transfers and the node
connection. `second-instance` now calls the same restore path, since focusing
a hidden window does nothing visible.

The context menu is not decoration. Under libappindicator -- how GNOME shows a
tray at all, via the AppIndicator extension -- `tray.on('click')` never fires;
the indicator only opens its menu. A tray whose sole affordance was a click
would be inert on the one desktop this targets. The click handler is kept for
desktops that do send it.

Menu labels come from the renderer with the IPC call: the locale files are the
interface's, the main process has no i18n, and a second string table is how two
of them start disagreeing. English fallbacks if none arrive.

The icon lives in src/, not build/: package.json `files` packages only `src/**`
and `ui/**`, so an icon under build/ is present in a dev run and missing from
every installed one. Monochrome, stroked, matching the nav glyph.

Verified on this host: Ubuntu GNOME with ubuntu-appindicators@ubuntu.com and
libayatana-appindicator3 present, so the indicator has somewhere to appear.
Not yet run end to end.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DtfG7z6wHWj8RKHCvxQtY1

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