fix(files): improve filter layout

[KurosawaGeeker]

Summary

• replace the native Thread selector with a compact, bounded dropdown
• show readable Thread titles and Agent names while keeping raw Thread IDs copy-only
• keep Agent and Thread filters aligned on wide screens and stacked on narrow screens
• centralize clipboard handling with an embedded-browser fallback

Why

Long Thread prompts made the native select expand into an unreadable full-width list. Raw Thread IDs also added visual noise without helping users identify a Thread.

Validation

• just check
• Files list model tests
• agent instruction prompt tests
• visible-copy scanner
• browser verification at narrow and 1600px-wide viewports
• verified long-title truncation, Thread selection, and copy confirmation

Generated files and data changes

• Generated files: no
• GraphQL codegen: no
• DB migrations or baseline changes: no
• Lockfile changes: no