TUI: Load a collection via sendra-core #94

Goal

Wire the TUI to real collection data.

Scope

• CLI arg (or default cwd lookup) for the collection file path, mirroring sendra-cli's resolution logic.
• Reuse .sendra/ project-local vs global config resolution exactly as sendra-cli does — no parallel implementation.
• On startup, load and parse the collection via existing sendra-core types.
• Parse/load failures shown as a readable in-app error screen, not a raw panic or silent exit.

Acceptance criteria

• Pointing the TUI at a real example collection (e.g. from examples/) loads it into AppState.
• Pointing it at a malformed YAML file shows a clear on-screen error instead of crashing.

Branch

feature/tui-load-collection

Part of the V1 TUI effort. Depends on Issue 2 (Elm skeleton). Locked order: Issue 3 of 15.