feat(v3): generate Linux hicolor icons

[mortenolsrud]

Summary

• add opt-in -linuxoutputdir and independent -linuxsizes flags to wails3 generate icons
• generate exact square PNGs with Go-native Catmull–Rom resizing, preserved aspect ratio, transparent padding, and deterministic encoding
• stage the complete icon set on the destination filesystem and use a recoverable directory swap so failed encoding never replaces valid output
• update fresh-project Taskfiles to track all seven default outputs and install them into matching hicolor directories for DEB, RPM, and Arch packages
• preserve the legacy single-icon nFPM mapping when update build-assets detects an older Taskfile that cannot generate the new files
• document the canonical-source/generated-file policy and custom-size packaging contract

Compatibility

The existing Windows/macOS options and ICO -sizes parsing are unchanged. Raw Linux generation is opt-in. Fresh projects receive the new Taskfile and package mappings; existing projects keep packageable legacy behavior until their preserved Taskfile declares the complete default Linux set.

The portable directory swap is recoverable rather than a lock-protected atomic exchange: readers may briefly see the output directory absent between renames, but they never see partially written PNGs, and the next invocation restores the previous complete set after interruption.

Verification

• go test ./internal/commands ./internal/packager -count=1
• targeted go test -race for Linux generation and build-asset compatibility
• go vet ./internal/commands
• Windows amd64 and Darwin arm64 command-test cross-builds
• generated nFPM content preparation checked for deb, rpm, and archlinux
• git diff --check

Closes #5917.
Related to #4025.