Audio/video media metadata: expose over EC, in the REST API, and in the File Details dialog #418

Note: This should be implemented after #417

Summary

aMule stores audio/video media metadata (length, bitrate, codec, artist, album, title) on
a file, but this information is not carried over the EC protocol, so neither the
amuleapi REST API nor the remote GUI can show it — and the desktop File Details dialog
doesn't display it either. This issue adds media metadata end-to-end in three parts.

Effort: HEAVIEST — the only issue that touches all three layers (unlike issues 01–04,
which are webapi-only). It requires an EC-protocol addition on the amuled side.

Background

The six media fields live on CAbstractFile as FT_MEDIA_* tags
(src/include/tags/FileTags.h:72-77) and are read via GetIntTagValue / GetStrTagValue:

GetMetaDataVer() (src/KnownFile.cpp:124) returns non-zero only once media has been
probed — use it to guard emission/display so files without metadata cost nothing.

Reference emitter (Kad/ed2k tag build): src/KnownFile.cpp:1260-1268. Reference reader
(these same tags in the search list): src/SearchListCtrl.cpp:265-271.

Part 1 — amuled / EC protocol

• Add new tag codes (e.g. EC_TAG_KNOWNFILE_MEDIA_LENGTH, _BITRATE, _CODEC,
  _ARTIST, _ALBUM, _TITLE) to src/libs/ec/cpp/ECCodes.h, plus their entries in the
  name-lookup switch in the same file.
• Emit them in CEC_SharedFile_Tag (src/ECSpecialCoreTags.cpp:214, the base ctor shared
  by both shared files and partfiles) from file->GetIntTagValue(FT_MEDIA_LENGTH/BITRATE)
  and file->GetStrTagValue(FT_MEDIA_CODEC/ARTIST/ALBUM/TITLE).
• Guard the whole block behind GetMetaDataVer() != 0 so files without probed metadata add
  no tags.

Because the emitter is the shared base ctor, both /shared and /downloads (partfiles)
get the tags automatically — no per-role code.

Part 2 — webapi (amuleapi)

• Decode the new tags in src/webapi/Refresher.cpp into a new FileSnapshot sub-struct
  (src/webapi/State.h).
• Expose an inline media object on both GET /downloads/{hash} and GET /shared/{hash}
  via the shared write helper (see the overview's two-endpoint model). Note: the
  GET /shared/{hash} detail endpoint is introduced by issue 01 — media is one of its
  fields.

"media": {
  "length_s": 5400,
  "bitrate": 1500,
  "codec": "h264",
  "artist": "...",
  "album": "...",
  "title": "..."
}

• Omit the media object entirely when the file has no metadata.

Part 3 — aMule desktop GUI (amule) + remote GUI (amulegui)

Show the metadata in the File Details modal, which currently has no media section.

• Add a new "Media Info" wxStaticBoxSizer (with IDC_FD_MEDIA_* label IDs) to
  fileDetails() in src/muuli_wdr.cpp, and declare the IDs in src/muuli_wdr.h —
  mirroring the existing "Intelligent Corruption Handling" box.
• Populate the labels in CFileDetailDialog::UpdateData() (src/FileDetailDialog.cpp:148-200)
  from m_file->GetIntTagValue(FT_MEDIA_*) / GetStrTagValue(FT_MEDIA_*), guarded by
  GetMetaDataVer().
• amulegui (CLIENT_GUI build): the remote GUI's m_file is a proxy populated from
  EC, so it must decode the Part-1 tags into the proxy in the partfile-update path
  (src/amule-remote-gui.cpp ~2073, alongside the existing source-names / comments / A4AF
  decode). Store them with AddTagUnique(CTag(FT_MEDIA_*, value)) so the same
  GetIntTagValue / GetStrTagValue calls used in UpdateData() work identically in both
  the monolithic and remote builds — no build-specific display code.

Acceptance criteria

• For a probed media file, GET /downloads/{hash} and GET /shared/{hash} return a
  populated media object; for a non-media file the object is absent.
• The File Details dialog shows a "Media Info" box with the six fields in both the
  monolithic amule and the remote amulegui.
• Files without metadata add no EC tags and show no (or an empty) Media Info box.