chore: UI improvments for sending prompt

[kevinkim-ogp]

Send prompt design improvments part 1

• Updates action name to 'Use Pair'
• Refine UI labels
• Hide RTE menu bar
• Improve field name readability (accept alphanumeric characters with spaces instead of underscores/hyphens), sanitises them to underscores internally

What changed?

• Field names for Pair response fields now accept letters, numbers, and spaces (e.g., confidence score) instead of underscores and hyphens . Spaces are automatically converted to underscores for storage, while the original user-entered name is preserved as originalFieldName.
• Shared outputFieldNameSchema, sanitizeOutputFieldName, and toSanitizedOutputFieldNamePair utility have been extracted into packages/backend/src/apps/pair/common/schema.ts and reused across both the send-prompt and process-image schemas.
• The send-prompt action has been renamed from Ask Pair to Use Pair, with updated field labels and descriptions to improve clarity (e.g., Describe what you want Pair to do, Define how you want Pair to structure what it extracts).
• The fieldName and fieldType subfield order in the response fields UI has been swapped so Type appears before Output name.
• The RTE menu bar is now hidden when customRteMenuOptions is an empty array, allowing keyboard shortcuts to still work while keeping the UI minimal.
• Data-out metadata labels now display spaces instead of underscores (e.g., confidence score instead of confidence_score).

How to test?

• Verify that the RTE is shown without the MenuBarand you can still format text using keyboard shortcuts
• Verify that column headers are shown only once at the top of the multirow-multicol for outputs
• Verify that you can create output names with alphanumeric characters and multiple whitespace
• Verify that field names containing underscores, hyphens, or special characters (e.g., field_name, field@name) are rejected with validation error.
• Verify that dataOut is shown with the user's input (e.g., Priority level, confidence score)

[kevinkim-ogp]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• refactor: create RTE with presets #1558
• chore: UI improvments for sending prompt #1556 👈 (View in Graphite)
• PLU-708 (chore): design improvements #1554
• feat: add maxFiles, multirow column labels #1553
• trunk/ai-actions

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