fix: avoid oversized web multi-delete requests

[uxjulia]

Summary

• Change the web file manager multi-delete flow to send one small delete request per selected item instead of one large paths payload.
• Keep the delete modal disabled while the batch is running and report partial failures without aborting the whole selection.
• Add an Unreleased changelog entry for the user-facing fix.

Root Cause

The browser was posting all selected paths as one URL-encoded JSON array to /delete. With larger selections, that request can exceed the small request/parsing budget on the ESP32-C3 before the firmware deletes anything. Splitting the batch in the browser keeps each request small and reuses the existing single-path delete handler.

Fixes #269.

Validation

• python3 scripts/build_web.py
• pio run -e simulator
• pio run -e tiny -j1
• git diff --check