feat(webserver): wire up priority selector on shared page - #210

The priority dropdown on the shared files page of the default template
was dead UI: it only offered Low/Normal/High and its OK button emitted a
'setprio' command that the backend never handled, so nothing was applied.

Template changes (amuleweb-main-shared.php):
- Expand the selector to the full set of priorities (Auto, Very low, Low,
  Normal, High, Very high, Release), with Auto as the first option.
- Wire the 'setprio' action: map the selected label to its priority
  constant (src/Constants.h) and dispatch the real 'prio' command via
  amule_do_shared_cmd($hash, 'prio', N).

Priority arrow logic (WebServer.cpp):
- GetHigherPrioShared/GetLowerPrioShared now walk only the manual scale
  (Very low <-> Low <-> Normal <-> High <-> Very high <-> Release) with
  clamped ends; the arrows no longer set or pass through Auto.
- Raising while in Auto leaves it at High, lowering leaves it at Low.
  Auto can now only be enabled from the selector, fixing the previous
  behavior where raising priority in Auto dropped the file to Very low.