Upload one locally-kept crash report from the player itself.

Deliberately does NOT delete on success. The caller deletes, through the existing file API, only after this reports the upload actually landed -- deleting the only copy of a crash report on an unverified "probably sent" is how the evidence disappears. For FPP's own web UI only (ShowCrashUploadDialog() in www/js/fpp.js, which asks the user before every send). Other apps and plugins should open FPP's Diagnostic Report instead. A -manual.zip report (built by POST /api/crashes/report) needs a JSON body of {"consent": true}: the user's answer in that dialog, never a value a caller sets on their behalf. If the player cannot reach the server (CanRetryFromBrowser), the same dialog has the browser post the file itself to the url and field that GET /api/crashes/uploadTarget returns. Error Code: consent-required (a -manual.zip report without {"consent":true}), not-found (no such report on this player, e.g. pruned by newer builds).