feat: add branch search and validation in header popover

[Dimillian]

Motivation

• Improve branch popover UX by making the branch list searchable and using the existing branch ordering (most-recent first) when presenting results.
• Allow quick creation of a branch from the search field and provide client-side validation and inline feedback to prevent invalid names.

Description

• Replaced the old create-toggle UI with a single searchable input that filters branches by substring and falls back to the most-recent 12 branches when the query is empty.
• Pressing Enter will either create a new branch (when the query has no exact match) or checkout an exact match (when different from the current branch), and the search is cleared after actions or when the popover closes.
• Added comprehensive client-side branch name validation rules (disallowing reserved names, spaces, invalid characters like ~, ^, :, ?, *, [, \, @{, .., .lock, leading/trailing /, trailing .) and surfaced validation messages in the popover.
• Added CSS for the search row, disabled create button state, and a small create hint (.branch-search, .branch-create-hint, and disabled .branch-create-button).

Testing

• Ran npm run lint and it completed successfully.
• Ran npm run typecheck (tsc --noEmit) and it completed successfully.

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Codex Task

[Dimillian]

@codex review

[chatgpt-codex-connector]

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