Detect read-only BLE filesystem via write probe (workaround for #376)

[makermelissa-piclaw]

Fixes #376 (workaround).

Problem

When CIRCUITPY is mounted read-only (USB MSC owns the filesystem), BLE file operations from the web editor fail silently — no error dialog, just unhelpful console errors and broken Save As / Upload / Rename / Move flows.

The web editor already has the plumbing for this case: fileHelper.readOnly(), the _readOnlyMode flag in the file dialog, and disabled Save As / Save+Run buttons in script.js. The probe is just short-circuited with an early return false, which is why it never fires.

The original detection relied on the firmware returning STATUS_ERROR_READONLY (0x05). Recent CircuitPython collapses that into the generic STATUS_ERROR (0x02) on some code paths (see adafruit/circuitpython#10972), so even bringing the old probe back wouldn't be enough to distinguish read-only from other errors purely by status code.

Fix

Drop the early return false and use a status-code-agnostic probe: attempt to write a hidden zero-byte file (/._ble_readonly_check) at the root and treat any failure as read-only. Cleanup of the probe file is best-effort and only attempted when the write succeeded.

This works on both old (0x05) and new (0x02) firmware, since it doesn't care which error code we got — only whether the write completed.

Effect

Once the probe correctly returns true:

• script.js: checkReadOnly() shows the existing warning: "Warning: File System is in read only mode. Disable the USB drive to allow write access."
• Save As and Save+Run buttons get disabled
• In the file dialog, Upload / New Folder / Rename / Move / Delete are all disabled by the existing _readOnlyMode guards

Verification

• npm run build clean (vite v8.0.10, 52 modules, ~2.7s)
• Probe is hidden filename (._ prefix), zero bytes, deleted immediately on success
• Probe path is the root /, which is always valid on CircuitPython, so no false positives from "Invalid Path"

Notes

• This is a client-side workaround. The real firmware fix (restore STATUS_ERROR_READONLY on read-only operations) is tracked in adafruit/circuitpython#10972. Even after that lands, this probe is still useful — it doesn't depend on a specific firmware version.
• Single file changed: js/common/ble-file-transfer.js (+19 / -6).

[makermelissa-piclaw]

Closing per discussion: rather than ship a client-side workaround for the readonly-FS misreport, we'll wait for the firmware fix in adafruit/circuitpython#10972 so the FAT layer reflects live USB host state. Leaving the code editor in its current state.

The companion read-side helpers (getRuntimeStatus / getReadOnlyReason) drafted on adafruit/circuitpython-repl-js are also being shelved for the same reason — diagnostic value isn't worth the API surface if we're not acting on it.