feat: create segment from CSV drawer

[Zaimwa9]

• I have read the Contributing Guide.
• I have added information to docs/ if required so people know about the feature.
• I have filled in the "Changes" section below.
• I have filled in the "How did you test this code" section below.

Changes

Adds the "New Segment — from a CSV list" drawer behind the segment sources modal, and makes the sources modal config-driven instead of hardcoded.

Flag-driven sources. The modal now reads its cards from the JSON value of the create_segment_with_external_sources flag:

[
  { "active": true, "name": "csv", "visible": true },
  { "active": false, "name": "amplitude", "visible": true },
  { "active": false, "name": "mixpanel", "visible": true },
  { "active": false, "name": "adobe_journey_manager", "visible": true }
]

visible: false hides a card, active: false renders it as a Beta fake door, active: true makes it real (currently only CSV has an implementation — an active source without one safely stays a fake door). Array order controls display order. Copy and icons stay in the frontend. Flag enabled with an empty or invalid value falls back to the manual drawer.

CSV drawer.

• Client-side CSV parsing (quoted fields, CRLF, BOM)
• Optional header row
• Identifier column selection with a highlighted 5-row preview
• Ignored-row warnings (empty/duplicate) and a blocking error when a column yields no identifiers
• Submission is wired in the follow-up PR

How did you test this code?

Unit tests for the CSV parser/extractor (common/utils/__tests__/csv.test.ts). Manually:

1. Set the JSON value above on the create_segment_with_external_sources flag and enable it.
2. Segments → Create Segment: modal shows the cards; non-active sources show the Beta chip and the request-access flow.
3. Click "From a CSV list" (active): the drawer opens; upload a CSV with/without headers, switch the identifier column, and check the preview, warnings, and the empty-column blocking error.
4. Disable the flag or set the value to []: the button opens the manual drawer directly.

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