Retarget the survey2ddi references: conversion is formtransform now

Part of the survey2ddi retirement. Plan: HANDOVER_SURVEY2DDI.md in the formtransform repo. Do this last, once CorrelAid/survey2ddi#5 and #6 have landed — the site should not announce a change before it is true.

What changed upstream

survey2ddi is no longer the CDL's survey converter. @correlaid/formtransform does every conversion (XLSForm → LimeSurvey TSV / DDI, LimeSurvey TSV → DDI / XLSForm, plus the response-data CSV) and the app is its browser front end. survey2ddi shrinks to the analysis-side DDI reader — reading a codebook back to label a dataframe.

Files naming survey2ddi as the converter

Do

• Point every conversion claim at formtransform or the app.
• Keep survey2ddi in the ecosystem picture, re-scoped to the analysis/reader role — it is still a published package with a real job, so deleting it from the flowchart would be wrong. Its phase in toc.json may need to move accordingly.
• Both language snippets stay in sync; the German flowchart labels are user-visible strings, not identifiers.
• Same pattern as the app's scope notice, #25.

Acceptance criteria

• No page credits survey2ddi with converting anything
• survey2ddi still appears, described as the DDI reader
• de/en snippets consistent
• Flowchart and workflow table agree with each other and with the tools page
• Site builds

Start with HANDOVER_SURVEY2DDI.md and confirm issues #5 and #6 have landed before editing the listed snippets, toc.json, workflow.astro, EcosystemFlowchart.astro, and content pages. Compare the German and English wording, then run the site build; done means the acceptance criteria pass and survey2ddi remains represented only as the DDI reader.