Refactor TUI profile modal state

[rmanibus]

Summary

• extract source-specific profile modal behavior into a dedicated helper type
• replace string-encoded modal field errors with a typed validation error
• make modal view generation side-effect free and add focused tests

Verification

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

[codecov]

Codecov Report

❌ Patch coverage is 58.20896% with 56 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cmd/cloudstic/cmd_tui_profile_source.go	53.62%	31 Missing and 1 partial ⚠️
cmd/cloudstic/cmd_tui_profile_form.go	63.07%	22 Missing and 2 partials ⚠️

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