signal-desktop-ma: force separate --user-data-dir

[vxfemboy]

Follow-up to #23. The shared-~/.config/Signal-directory tradeoff documented in that PR (declined isolation, accepted the risk) broke for real within hours: the multi-account fork's schema cap (1730) is behind official Signal-Desktop's current schema (which reached 1740 once the official package was updated), and a profile touched by the newer app becomes unopenable by the older one. No data was lost — SQLite migrations are additive and Signal's own startup guard refuses to touch a database from a newer schema rather than risk corrupting it — but access was blocked until the official package caught up.

Change

do_install() no longer symlinks /usr/bin/signal-desktop-ma straight to the binary. It's now a wrapper that forces --user-data-dir="$HOME/.config/Signal-MA":

#!/bin/sh
exec /usr/lib/signal-desktop-ma/signal-desktop --user-data-dir="$HOME/.config/Signal-MA" "$@"

Verified this is the correct mechanism: Electron's native --user-data-dir CLI switch is read before any app-level JS runs, and this fork's own config-based override (storagePath/storageProfile, traced in app/user_config.main.ts during the original design work) only fires if those config keys are explicitly set — they aren't by default, so nothing in the app overrides the forced path.

Test plan

• xlint signal-desktop-ma clean
• Rebuilt, installed
• /usr/lib/signal-desktop-ma/signal-desktop --user-data-dir=<path> --version confirmed the flag works against the real binary directly
• /usr/bin/signal-desktop-ma --version (absolute path, bypassing an unrelated pre-existing /usr/local/bin/signal-desktop-ma on this host that shadows it) confirms userData: $HOME/.config/Signal-MA