Add TUI check action and health service

[rmanibus]

Summary

• add a check action to the interactive TUI
• enrich the selected profile pane with store health and backup freshness
• route TUI orchestration through a shared internal/app service backed by a named backend interface
• update the user guide and RFC 0012 to match the current implementation

Closes #213.

Testing

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

[codecov]

Codecov Report

❌ Patch coverage is 45.68345% with 151 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/tui_runtime.go	3.70%	49 Missing and 3 partials ⚠️
internal/tui/shell.go	31.57%	35 Missing and 4 partials ⚠️
internal/app/tui_service.go	59.37%	17 Missing and 9 partials ⚠️
internal/tui/dashboard.go	63.23%	19 Missing and 6 partials ⚠️
cmd/cloudstic/cmd_tui_activity.go	74.07%	5 Missing and 2 partials ⚠️
cmd/cloudstic/cmd_tui_input.go	75.00%	1 Missing and 1 partial ⚠️

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