Title: [Feature]: Kanban assignee should be a dropdown select instead of free text input

Problem or Use Case

The Kanban WebUI currently uses a free text input for assigning tasks to profiles. This creates several UX problems:

1. Case sensitivity errors - Users must type profile names exactly (lowercase), but the WebUI displays them in uppercase due to CSS styling. This causes profile validation failures and task crash loops (see Kanban WebUI silently converts input to uppercase, causing profile mismatch crash loops with no notification #21320).

2. No discoverability - Users must already know their profile names. There's no way to see available profiles without switching to the CLI (hermes profile list).

3. Copy-paste workflow - To avoid typos, users currently need to copy profile names from another source and paste them in, which is cumbersome.

4. Silent failures - When a profile name doesn't match exactly, the dispatcher fails with no clear notification to the user.

Proposed Solution

Replace the free text assignee input with a <select> dropdown that:

• Populates from the available profile list (fetched from backend or config)
• Shows profile names exactly as they exist (lowercase)
• Optionally includes profile descriptions/model info as secondary text
• Prevents invalid assignments at the UI level

Alternatives Considered

• Autocomplete/typeahead input - still allows typos, more complex to implement
• Input validation with error message - reactive rather than preventive
• Auto-lowercase on submit - doesn't solve discoverability or typo problems

Additional Context

Current profiles require exact matching against [a-z0-9][a-z0-9_-]{0,63} pattern. A dropdown would eliminate an entire class of user errors and make the Kanban board more accessible to new users.

Contribution

Happy to test or provide feedback on a PR.