v0.8.12: attachments in the composer, picker + drop + paste

A prompt can carry files now, and they come in the three ways a desktop user
expects: the 📎 button in the composer (QtQuick.Dialogs' FileDialog), files
dropped anywhere on the window (a DropArea over it, with an accent border and
"drop to attach" while something is dragged), and an image on the clipboard
(Ctrl+V). Text keeps the editor's own paste: the composer asks
AgentCore::clipboardHasImage() first and only takes the keystroke when there is
an image.

AgentCore (app/src/core/AgentCore.*):
- pending is a property now (QStringList) fed by cst_agent_pending_describe();
  attachFile() and attachClipboard() wrap cst_agent_attach_file() /
  cst_agent_attach_bytes(), clearPending() wraps cst_agent_pending_clear(), and
  seesImages() reports whether the model is shown images at all.
- the engine's description says what the model gets ("png 12.0 KB"), not where
  it came from, so the window remembers the names it attached and puts them in
  front: "note.txt — text 21 B".
- pendingChanged() fires on attach, on clear and when a turn ends (the prompt
  consumed the list), which is what makes the chips move.

Engine (src/attach.c):
- cst_attach_describe() printed every sub-megabyte size as "%.0f KB": a 21-byte
  text file came out as "text 0 KB". It is "B", "KB" with one decimal or "MB"
  now, and test_attach still passes.

QML: the composer shows one chip per pending file with "clear all"; the hint
line mentions 📎/Ctrl+V and says when the model is text-only (images then
travel as notes); Main.qml owns the DropArea and the drag feedback.

Verified: 44/44 UI checks (8 new: attach ok, one pending, the name is shown, a
missing file is refused, one chip in the composer, clear all empties it, the
clipboard image is seen and attached), --selftest, make check rc=0 (test_attach
included), and screenshots of the chip row.

Version 0.8.12.

include/cst_version.h

@@ -3,7 +3,7 @@
 
 #define CST_VERSION_MAJOR 0
 #define CST_VERSION_MINOR 8
-#define CST_VERSION_PATCH 11
+#define CST_VERSION_PATCH 12
 
 #define CST_VERSION_STR_(x) #x
 #define CST_VERSION_STR(x) CST_VERSION_STR_(x)