ci: configure golangci-lint and gofumpt

[thecentinol]

Summary

Set up golangci-lint and added a CI job to verify formatting using gofumpt.

Changes

• Configured golangci-lint.
• Added a CI lint job running golangci-lint and gofumpt checks.
• Added lint and format targets to the Makefile.
• Resolved all existing linter warnings and reformatted the codebase.

Note: Most changes across internal/ are automated fixes applied by gofumpt. Two linter warnings in keybindings were intentionally left unresolved to prevent merge conflicts with feature/keybinds-modal (where they will be addressed).