fix(export): server-accurate export size estimate (incl. media)

[DavidBabinec]

What

The Estimated size line in the Export dialog was a client-side guess that:

1. Ignored media entirely — mediaAssetCount was hardcoded to 0 (ExportDialog.tsx:205, comment: "caller doesn't provide a cheap count yet"), so toggling Media files never changed the number.
2. Underestimated content ~10× — flat constants (8 KB shell + 1.5 KB/row) that don't model real page-trees/styleRules.

This read as "media isn't being exported" — but the export is correct; only the estimate was wrong. (Verified separately: a real export of a 19-image site embeds all binaries as base64 and round-trips on import. See the analysis in the conversation that produced this PR.)

Fix — one source of truth

Replace the heuristic with an exact, server-computed estimate that reuses the export's own selection logic so the number can never drift from the real download:

• New POST /admin/api/cms/export/estimate — runs the identical table/row/media selection as the real export, but computes the byte size instead of streaming the bundle. It never reads media files off disk or base64-encodes them: it serializes the real selection with empty bytesBase64 strings and adds each asset's analytic Base64 length (4·⌈n/3⌉). Base64 is ASCII, so this equals the real bundle byte length exactly.
• export.ts refactor — buildBundle + the media-metadata mapping are shared between the download and estimate paths, so they can't diverge. Behavior of the existing download path is unchanged.
• useExportEstimate — now fetches the endpoint (debounced 250 ms, cancellable via AbortController) and formats the result; the dialog mirrors the exact ExportRequest it will send. The previous value stays on screen while a new estimate is in flight (no flicker); failures show unavailable.

Verification

• bun run build ✓ · bun run lint ✓ · bun test ✓ — 5439 pass, 0 fail
• New tests assert estimate === real download byte length for the no-media, no-shell, and embedded-media cases (cmsTransferExport.test.ts), plus a dialog test that the estimate re-fetches and grows when Media is toggled (exportDialog.test.tsx).
• Live, against a real site (19 images, ~69 MB), on the running server:
  • estimate without media = 183,618 B (~179 KB)
  • estimate with media = 96,769,329 B → actual download with media = 96,769,329 B (byte-identical)
  • Dialog screenshot confirmed: Media off → ~179 KB, Media on → ~92.3 MB.

Note: the pre-push hook surfaces 6 react-doctor warnings on ExportDialog.tsx (lines 142/144/165/264/279 — the useState cluster, reset-on-open pattern, and <label>/Switch markup). All are pre-existing patterns unrelated to this change; they only appear because the file is in the diff. bun run lint (the CI gate) passes.

🤖 Generated with Claude Code