Add search panel support in CSV viewer

[Copilot]

PR Checklist

• Functionality has been tested, no obvious bugs
• Code style follows project conventions
• Documentation/comments updated (if applicable)

Brief Description of Changes

Adds search functionality to the CSV viewer with Ctrl+F, matching the existing TextViewer UX pattern.

New files:

• Controls/SearchPanel.xaml / .cs - Search UI control with text input, match count, prev/next buttons, match case toggle

Changes to CsvViewerPanel:

• Ctrl+F opens search panel, Escape closes it
• F3/Shift+F3 for find next/previous navigation
• Enter/Shift+Enter within search box for navigation
• Case-sensitive search option
• Matching cells highlighted with semi-transparent yellow
• Efficient single-cell highlight tracking (no full grid iteration)

Related Issue (if any)

Closes feature request: support search panel in CSV viewer

Additional Notes

Search panel styling matches existing TextViewer search panel for visual consistency.

Original prompt

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This section details on the original issue you should resolve

<issue_title>[feature request] support search panel in CSV viewer</issue_title>
<issue_description>support search panel in CSV viewer</issue_description>

<agent_instructions>support search panel in CSV viewer</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes [feature request] support search panel in CSV viewer #1824

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