TUI: render selection actions as clickable buttons

[rmanibus]

Summary

• render selection-pane actions as button-like rows with keyboard hints
• make enabled action rows clickable through the existing mouse path
• keep disabled actions informational and non-clickable

Testing

• env GOCACHE=/tmp/cloudstic-gocache go test -count=1 ./internal/tui ./cmd/cloudstic
• env GOCACHE=/tmp/cloudstic-gocache GOLANGCI_LINT_CACHE=/tmp/cloudstic-golangci-lint golangci-lint run ./internal/tui ./cmd/cloudstic

[codecov]

Codecov Report

❌ Patch coverage is 70.90909% with 32 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/cmd_tui_input.go	32.25%	19 Missing and 2 partials ⚠️
internal/tui/shell.go	86.07%	8 Missing and 3 partials ⚠️

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