feat: enforce configured file-size limits across all local upload paths

[bdart]

Summary

Closes ERMAIN-34.

Prevents every local File larger than the runtime-configured per-file upload limit from starting an upload, regardless of how the file entered the UI. The configured limit (maxSizeBytes / maxSizeFormatted from useUploadFeature()) is enforced consistently — no hard-coded 20 MB anywhere in client code.

What changed

New utility — frontend/src/utils/validateFileSizes.ts

Pure, side-effect-free preflight function. Accepts File[] and maxSizeBytes; returns a discriminated union with { valid: true } or { valid: false; oversizedFiles: File[] }. One byte above the limit → invalid; exactly at the limit → valid.

API-facing upload hooks (correctness boundary)

File	Change
useFileDropzone.uploadFiles	Size preflight before setUploading, silent-chat creation, FormData, or fetchUploadFile.
useStandaloneFileUpload.uploadFiles	Imports useUploadFeature; size preflight before upload state or network request; UploadTooLargeError now always receives maxSizeFormatted (fixes the Maximum size: — regression on 413 responses).
useChatFileSources.handleSelectedFiles (onSelectFiles)	Size preflight before performDiskUpload protects the host/custom File[] path used by Outlook/Teams add-menu.

Selection surfaces (UX early-rejection)

File	Change
FileUploadButton	Reads maxSizeBytes from useUploadFeature; passes to useDropzone; adds onError prop; routes file-too-large rejections via onError.
FileUpload (buttonOnly)	Extracts setError from useFileDropzone and passes it as onError to FileUploadButton.
useConversationDropzone	Adds optional maxSize, maxSizeFormatted, and onError props; surfaces file-too-large rejections immediately via onError.
AssistantFileUploadSelector	Reads limit from useUploadFeature; passes maxSize to its hidden disk-picker dropzone; routes file-too-large into its cloudLinkError state.
Chat.tsx	Passes maxSize, maxSizeFormatted, and setUploadError as onError to useConversationDropzone.
OutlookAddinChat + AddinChatCore	Same useConversationDropzone wiring for both add-in conversation hosts.

Outlook direct email-source upload (AddinChatInput)

• After resolveSelectedFilesForSend(), validates file sizes with validateFileSizes before constructing FormData or calling fetchUploadFile. Oversized files set emailUploadError and skip the upload (message is still sent without the file IDs; chips remain for retry).
• In the catch block, isUploadTooLarge(error) maps HTTP 413 to UploadTooLargeError(maxSizeFormatted) instead of silently logging.
• emailUploadError is merged into chatInputProps.uploadError and surfaced through AddinChatInputCore → ChatInput.

Library exports

UploadTooLargeError, isUploadTooLarge, and validateFileSizes are now exported from @erato/frontend/library so add-in packages can consume them directly.

Test environment fix

Added frontend/src/__mocks__/desktop-sidecar-protocol.ts stub + vitest alias so tests that import through the library chain don't fail on the missing native package.

Tests added / updated

File	Coverage
utils/__tests__/validateFileSizes.test.ts	8 tests: boundary conditions (−1 byte, exact, +1 byte), batch rejection with one oversized file, multi-oversized batch, metadata (original File reference returned)
hooks/files/__tests__/useFileDropzone.test.tsx	6 new tests: size preflight blocks fetchUploadFile and chat creation, sets UploadTooLargeError with formatted limit, accepts exact-limit file, rejects mixed batch, isUploading stays false
hooks/files/__tests__/useStandaloneFileUpload.test.tsx	7 new tests: preflight rejection, formatted limit (no —), exact-limit accept, mixed-batch rejection, isUploading stays false, HTTP 413 with numeric and string status
hooks/files/__tests__/useConversationDropzone.test.tsx	6 new tests: backward-compatible no-maxSize mode, maxSize forwarded to useDropzone, file-too-large → UploadTooLargeError via onError, no uploadFiles call on rejection, valid files still upload
hooks/files/__tests__/useChatFileSources.test.tsx	4 new tests: onSelectFiles preflight rejects oversized, formats limit correctly, accepts exact limit, rejects mixed batch
components/ui/FileUpload/__tests__/FileUploadButton.test.tsx	4 new tests: maxSize from feature config passed to dropzone, file-too-large → UploadTooLargeError via onError, no performFileUpload on rejection, no onError on clean drop

Scope not covered in this PR (per issue)

• Cloud linking via fetchLinkFile (browser does not possess remote bytes)
• Audio transcription/dictation WebSocket protocols
• Backend limit changes
• File compression
• Automatic retry of oversized files

Testing

All affected test suites pass (vitest run). The FileUpload component tests, Chat tests, provider tests, and utility tests are unaffected.

Linear Issue: ERMAIN-34