Modernize Graphlink Composer to a production-grade request surface

[dovvnloading]

What changed

• Replace the separate attach/editor/send row with a unified Composer shell.
• Add visible graph context, attachment summary, provider route, request status, and accessible action labels.
• Extract draft/request identity and lifecycle transitions into ComposerController.
• Add stale-request protection so late worker responses cannot overwrite a newer request.
• Preserve draft text and attachments when attachment reads, provider requests, or cancellation fail.
• Keep large paste and dropped text in the prompt draft by default; explicit attachment remains available.
• Add provider-mode status in the Composer and package the new module.
• Add five Composer controller/UI regression tests.

Why

The previous Composer split visual controls from the request lifecycle. Context was implicit, send/recovery state was distributed across ChatWindow and WindowActionsMixin, and failed sends cleared user work. The new shell gives the user a single, inspectable interaction surface while keeping the migration compatible with the existing graph worker contracts.

Validation

• QT_QPA_PLATFORM=offscreen python -m pytest graphlink_app/tests -q
• Result: 419 passed, 1 warning.
• python -m compileall -q graphlink_app
• git diff --check

The local-only proposal at doc/COMPOSER_SOTA_REFACTOR_PLAN.md remains ignored and is intentionally not part of this PR.