[Tock Studio] add Markdown ZIP export with GFM-aware heading demotion and RAG-oriented structure (Dercbot 1836)

[rkuffer]

Closes #2015

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Summary

This PR adds Markdown as a third export format to the DataExportComponent, alongside the existing JSON and CSV options. Note that DataExportComponent is currently only used for FAQ export — this context drove the design decisions around heading structure, GFM detection, and RAG-oriented output.
Each entry in the exported collection produces an individual .md file; all files are bundled into a .zip archive. The implementation is oriented toward RAG pipeline ingestion and handles the case where field values already contain Markdown or GFM content.

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Changes

data-export.component.ts

• Added Formats.md to the Formats enum
• Added @Input() defaultColumns?: string[] — optional ordered list of pre-checked columns; columns listed here appear first and are checked by default, remaining columns are appended unchecked
• Added @Input() mdHeadingPrefix?: string — optional prefix for the H1 heading of each generated file (# {prefix}: {title}, # {prefix}, or # {id} depending on available data; no H1 if none available)
• Refactored column FormGroup construction in ngOnInit to use an intermediate Map, enabling defaultColumns ordering to be applied after all groups are built
• Fixed canIncreaseColumnIndex bound: now uses this.columns.length instead of this.columnNames.length, which was incorrectly capping reordering for virtual columns produced by deepening strategies
• Added buildMarkdownFileName(): cross-OS safe filename sanitization (diacritics, forbidden characters, Windows reserved names, 200-char max length), combining title and id with index-based fallback when neither yields a usable string after sanitization
• Added buildMarkdownHeading(): builds the H1 line from mdHeadingPrefix and/or title/id, returns null when no data is available (no H1 emitted)
• Added looksLikeMarkdown(): remark/GFM-based AST detection with a tiered confidence model — high (code, link, table), medium (heading, blockquote, strikethrough), low (list, strong) — requiring at least one high signal, two medium, or one medium + one low
• Added demoteMarkdownHeadings(): AST-based heading demotion via remark-parse + remark-gfm + visit — increments all heading depths by 1 (capped at 6); code block contents are opaque AST nodes and are never affected
• Added serializeValueToMarkdown(): async serialization dispatcher — bullet list for arrays, **key**: value for objects, Markdown detection + heading demotion for strings, passthrough otherwise; empty results skip their section heading entirely
• Added generateMarkdownForEntry(): async, builds the full .md content for one entry
• Markdown ZIP generation uses Promise.all for concurrent entry processing, chained with zip.generateAsync, with a .catch that resets loading and surfaces a danger toast on failure
• Added two shared readonly remark pipeline instances (remarkParser, remarkSerializer) to avoid per-call unified() instantiation
• Column validator (validateColumnsFormArray) now also activates for Formats.md

data-export.component.html

• Added Markdown radio option to the format selector
• Extracted the column selection block from the CSV-only *ngIf into a shared ng-container conditioned on csv || md
• CSV delimiter options remain isolated in their own conditional block

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Dependencies

Added jszip depency.
The following already present packages are now imported:
unified, remark-parse, remark-stringify, remark-gfm, unist-util-visit (transitive — explicit declaration recommended).

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Testing notes

• Export a collection where some entries have no content for selected columns → empty sections should be absent from output
• Export a collection where answer contains GFM (headings, tables, lists) → heading levels should be demoted, structure preserved
• Export a collection where entries have titles with accented characters, special characters only, or no title/id at all → filenames should be valid and unique
• Export with defaultColumns containing a locale-specific column (e.g. answer.i18n.en) on an app that does not support that locale → column silently absent, no error
• Simulate a ZIP generation failure → loading resets, danger toast visible