fix(windows): use os.homedir() fallback so dmux starts on Windows

[pushp1997]

Summary

Fixes #85.

On Windows, process.env.HOME is undefined (Windows uses USERPROFILE). At src/index.ts:1148:

const homeDmuxDir = path.join(process.env.HOME!, '.dmux');

The ! non-null assertion lies to the type checker. At runtime, path.join(undefined, ...) throws TypeError: The "path" argument must be of type string. Received undefined. The exception is swallowed by .catch(() => process.exit(1)) at the bottom of init(), so Windows users see dmux silently exit with code 1 and never get past migrateOldConfig().

This PR replaces process.env.HOME! with process.env.HOME || os.homedir(), matching the safe pattern already used elsewhere in the codebase (src/utils/onboarding.ts, src/utils/openRouterApiKeySetup.ts, src/utils/tmuxConfigOnboarding.ts).

Changes

• src/index.ts:
  • Add import os from 'os';
  • Replace process.env.HOME! with process.env.HOME || os.homedir() in migrateOldConfig()

Note on src/server/embedded-assets.ts

The same buggy line exists at src/server/embedded-assets.ts:1374 inside a template literal. That file has a DO NOT EDIT MANUALLY - Generated by scripts/embed-assets.js header, so I left it alone in this PR. Whatever generator produces it should pick up this change on the next regeneration. Happy to include a follow-up if you'd like it patched manually in the meantime.

Test plan

• On Windows 11 / PowerShell 7, with the same patch applied locally to the installed dmux, dmux now successfully writes dmux.config.json and proceeds past migrateOldConfig() (exit 0 in non-TTY harness, ink UI launches in real terminals).
• On macOS/Linux: behavior unchanged because process.env.HOME is set, so the || short-circuits before os.homedir() is even called. (No CI run yet from my fork.)