Condenser: platform/RPC shim mapping @decky/api to condenser

[mdeguzis]

Route our backend calls through condenser.plugin.useSend instead of @decky callables, and navigation through condenser.nav. Reuse the httpPlatform/deckyPlatform split from dev-next so the same lib code runs on Decky and condenser.

useSend(pluginKey)(action, data) maps to a Node backend export. Map our callable names to backend.ts exports. Navigation: Router/Navigation calls map to condenser.nav navigate/back and the page/route surface.

[mdeguzis]

Platform/RPC shim backbone is in. Added src/lib/condenserPlatform.ts implementing our existing Platform interface (the same one deckyPlatform/httpPlatform implement) against the condenser global. call() routes through condenser.plugins.callPlugin (the non-hook form of useSend), so shared src/lib code works outside React. fetchNoCors proxies to a Node fetchProxy backend action since the CEF frontend context blocks cross-origin fetch. toast and showModal map to condenser.ui.

frontend.tsx now calls initPlatform(condenserPlatform) and routes the Page through platform(). Built frontend.js has zero @decky references and routes RPC via callPlugin. Next: getFocusedAppId real source and the showModal closer gap, then wire the actual tabs.