fix(windows): remove dead installer code from the shipped binary

[PathGao]

Problem

The code that triggered Trojan:Win32/Wacatac.B!ml was removed in #481, but the setup.exe (NSIS installer) is still being flagged by Windows Defender — see this comment on #466.

Three things that still look like an installer to a behaviour classifier were left in the binary:

1. forward_legacy_uninstall() — answered --uninstall from pre-2.7 custom installs. The NSIS template writes UninstallString = uninstall.exe unconditionally, and the auto-updater's /UPDATE pass runs Section Install, so every install that has ever updated through 2.7.1 already has the corrected registry entry. The hooks.nsi macro also drops stray --uninstall entries from both hives as a backstop.

2. is_installer_mode detection — checked for --install or "installer" in the exe name. No shipped artifact has triggered either since build: harden release inputs and isolate test bundles #355 stopped publishing an installer-named file, and fix(windows): remove the bundled installer and uninstaller (#466) #481 removed Installer.svelte, install_app, and the invoke_handler registrations that were the only consumers of the resulting window label.

3. The "installer" window label in capabilities — a window that no code can create any more.

None of these do anything observable today, but a scanner that reads the file rather than tracing reachability still sees them.

Change

• Delete forward_legacy_uninstall() and its call site in run()
• Delete is_installer_mode detection, hardcode label to "main"
• Delete the is_installer_mode window-sizing block
• Remove "installer" from capabilities/default.json
• Update the test that asserts the capability window list

Verification

npm test — 722 pass / 5 fail (same as master; the 5 are pre-existing Node.js v25 localStorage incompatibility)