Harden composer crash paths and large-paste attachments

[dovvnloading]

Summary

• Harden Qt popup and QWebEngine shutdown lifecycles.
• Fix Ollama settings signal initialization ordering.
• Restore large-paste attachment staging so long code/text is represented as context instead of being dumped into the composer.
• Add regression coverage for popup lifecycle safety, settings initialization, bridge routing, and paste behavior.

Root cause

The recurring failures came from multiple independent lifecycle defects: popup event filters and queued callbacks could run after their Qt wrappers had started closing, Ollama model selection could emit before its paired input existed, and the React composer bypassed the legacy large-paste attachment path. WebEngine shutdown also lacked an explicit stop/hide phase before Qt teardown.

Validation

• python -m pytest graphlink_app/tests: 482 passed
• python -m compileall ... graphlink_app: passed
• npm.cmd run typecheck: passed
• npm.cmd run build: passed
• Controlled QWebEngine create/load/shutdown smoke test: exited 0

The patch remains conservative: it preserves the existing bridge contract and native popup ownership while making teardown idempotent and defensive.