refactor(sections-editor): dedupe basename/extension helpers between image and file fields

[pedrofrxncx]

Source: C1 code reduction — found while auditing sections-editor/fields for duplication.

ImageField and FileField each defined byte-identical basename()/extension() helpers (URL → display filename / lowercase extension, used to render the preview file name and extension badge). Collapsed both into a new local module fields/media-filename.ts and import from there.

Payoff: one copy to maintain instead of two that can silently drift; net -30/+2 across the two field files (+13-line new shared module).

Behavior: unchanged — the extracted functions are copied verbatim, only their location changed, so ImageField/FileField render identically.

Reviewer check: cd apps/mesh && bunx tsc --noEmit (green). No existing unit test covers these pure UI-formatting helpers (no trust boundary — they only affect a displayed filename), so no test was added per the coverage-gate policy.

Locally verified: bun run fmt (no changes) and bunx tsc --noEmit in apps/mesh (green). Full CI (lint, full test suite) validates the rest.

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Summary by cubic

Deduped filename helpers in the sections editor by extracting identical basename and extension logic from ImageField and FileField into a shared module. No UI or behavior changes.

• Refactors
  • Moved helpers to apps/mesh/src/web/components/sections-editor/fields/media-filename.ts and updated imports in image-field.tsx and file-field.tsx.
  • Reduces duplication and keeps filename/extension rendering consistent.

^{Written for commit 6d4f5b5. Summary will update on new commits.}