Improve debug log path resolution and keypair save error handling to make behavior reliable in non-standard environments and avoid misleading success feedback.

Changes in this commit:

- shared/debug: replace HOME-based cache path construction with os.UserCacheDir().

- shared/debug: return a wrapped error when cache directory cannot be determined.

- keypaircreate: handle .pub write failures explicitly instead of ignoring errors.

- keypaircreate: preserve partial-save state when private key write succeeds but public key write fails.

- keypaircreate: track public-key save outcome and only render public-key success when it actually succeeded.

- keypaircreate: reset save status before each save attempt to avoid stale UI state.

User-visible impact:

- Debug logging now uses the platform cache directory consistently.

- Keypair save UI no longer shows contradictory success/error messages.

- Users are clearly informed when only the private key was saved.

Validation:

- cd src && make test

- cd src && make build

- cd src && make vet

Fixes #101

Fixes #102