app: the view mode is a type, and 'disasm' is gone

_active was a bare string with 49 comparisons across four modules and a fifth
value nobody meant to keep. "disasm" was assigned on exactly one path -- a
decompile that failed with nowhere to return to -- and named the same widget as
"listing". Four sites understood it; five compared against "listing" alone and
silently took the wrong branch:

  * Tab out of a failed decompile set "listing" instead of "decomp", so the
    first press appeared to do nothing.
  * rpc.py carried a workaround for a mode change that never arrived, keyed on
    being ALREADY in the ghost state -- so it fired in the rare case and not in
    the common one. Now keyed on LISTING, which is the case that happens.
  * drive.py asked the socket to show it "disasm", a value the app will now
    never report, and would have toggled twice and given up.

ViewMode is a StrEnum on purpose: _active goes straight to drivers as
cursor.kind and the pilot compares it to plain strings, so members being strings
keeps every payload and comparison working. What it buys is one place that says
which modes exist, and an AttributeError instead of silence on a typo.

Read it through is_listing/is_decomp/is_hex/is_graph/in_code rather than ==.
The bare comparisons are what let the ghost hide, and they are what the next
mode would have to hunt down -- adding "graph" already cost one crash that way
(_active_code_view returning None when a prompt closed).

view_modes_all_handled walks the enum and asks the app the questions it asks
itself. Verified it bites: adding a fifth unhandled member fails it twice.

746 checks, 142.3s.

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