Find microSD cards on Linux, through the mount table (#106)

[ww8l]

Closes #106 — and it is no longer read-from-source: Tim reproduced it on a real Linux box, where the app detects no card and he has to navigate to it by hand.

The bug

mounted_roots() split two ways — Windows, or else — and Linux landed in the else:

std::fs::read_dir("/Volumes")          // macOS only
    .map(|rd| rd.flatten().map(|e| e.path()).collect())
    .unwrap_or_default()               // Linux: Err -> empty, silently

/Volumes does not exist on Linux, so the list was always empty. It is the only source of card locations, so no microSD card was ever detected: FT5D, ID-52 and TH-D75 all unusable on a platform with published installers, with nothing indicating why. Cable radios were unaffected.

The fix

Linux has no single directory to point at — udisks2 uses /media/<user>/<label> on Debian/Ubuntu and /run/media/<user>/<label> on Fedora/Arch, and a hand-mounted card is wherever the operator put it. So instead of guessing at conventions, read /proc/mounts and filter by filesystem type: every radio here writes its card as FAT or exFAT, and no Linux system volume is either (fuseblk included, because exfat-fuse reports itself that way).

• /boot is dropped. The EFI system partition is vfat and every UEFI machine has one, so a plain "FAT means removable" rule would offer /boot/efi as a card on every Linux install.
• Mount points are unescaped. The kernel writes space/tab/newline/backslash as three-digit octal, so a card labelled MY CARD arrives as MY\040CARD and a path built from it verbatim would not exist. Unescaped bytes-first so an accented label survives; a backslash that is not a valid escape is left alone rather than guessed at.
• The three-way split is explicit #[cfg] arms, with compile_error! for anything else. A fourth platform now fails to build rather than silently inheriting another OS's convention — that inheritance is this bug.

Verification

• npm run ci green on macOS: 433 tests, clippy -D warnings clean.
• Three new tests parse a real Ubuntu mount table: the card is found under /media/<user>, /run/media/<user> and a hand mount under /mnt; /, /proc, /sys, a tmpfs and /boot/efi are all rejected; malformed lines are skipped rather than half-parsed. The parser is compiled into the tests on every OS, so this is checked on the machines that do not have the bug.
• Not yet confirmed against the Linux box itself — the mount table there has not been read, and no Linux build of this branch has been run. CI's ubuntu leg proves it compiles, not that it finds Tim's card.

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https://claude.ai/code/session_013cZgbB4FwqZ3Z4cdHt6vMH