fix(build): sync frontend assets to embed dir when frontend:dir is custom

[leaanthony]

Summary

• When frontend:dir in wails.json is set to a non-default directory, wails build now correctly syncs the built frontend assets to the //go:embed directory
• The go:embed directive in main.go is hardcoded (e.g., //go:embed all:frontend/dist), so when the frontend is in a custom directory, the assets end up in the wrong place at compile time
• After BuildFrontend, a new syncFrontendAssetsToEmbedDir step copies assets from <custom-frontend>/dist to frontend/dist
• No-op when frontend:dir is the default "frontend"

Test Plan

• v2/pkg/commands/build/sync_test.go contains 4 tests:
  • TestSyncAssetsToEmbedPathsCopiesFromCustomDir — verifies assets are copied from custom dir to embed dir
  • TestSyncFrontendAssetsNoOpForDefaultDir — verifies no copying when frontend:dir is default
  • TestSyncFrontendAssetsNoOpForNilProject — verifies nil safety
  • TestCopyDir — verifies the recursive directory copy utility

Related Issue

Fixes #5034

Notes

• Does not modify //go:embed directives (that would require rewriting user source code)
• For frontends outside the project directory, this fix works for relative paths. Absolute paths outside the project remain unsupported due to go:embed limitations.
• This is a v2-only fix.

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[taliesin-ai]

Superseded by #5251 (build now syncs frontend dist into the embed target). This was an earlier WIP for the same fix; closing as done.