v0.8.14: icons from the desktop's theme (Breeze), readable text, composer frame

The window drew its icons with unicode glyphs -- a clip emoji for attach, a
gear, a hamburger, a chevron, a multiplication sign for delete. Those render
in whichever font happens to cover them (colour emoji in a grey toolbar) and
are a box on a machine without that glyph. They now come from the icon theme,
the way every other desktop app gets them.

Icons
- app/src/ui/ThemeIcons.{h,cpp}: a QQuickImageProvider for
  image://csttheme/<name>?color=%23rrggbb -- QIcon::fromTheme() in the
  desktop's theme (whatever the platform theme set, else breeze-dark, breeze,
  hicolor), tinted with QPainter's SourceIn and rasterised at the requested
  size and the screen's device pixel ratio. Qt only builds its theme search
  paths from XDG once a theme name is set, so the provider fills them itself
  (GenericDataLocation/icons, ~/.icons and, on NixOS,
  /run/current-system/sw/share/icons, for a shell started outside the desktop
  session). An unknown name yields a null image, which QML reports as
  Image.Error.
- app/qml/ThemeIcon.qml: the one component that asks for an icon. Every use
  still names the old character as `glyph:`, so a host with no icon theme
  degrades to exactly what the window looked like before instead of going
  blank. installThemeIcons() is called by main() and by --uitest, so the test
  loads the same icons the app does.
- Replaced: nav (application-menu), settings (settings-configure), attach
  (mail-attachment), copy (edit-copy), session delete (edit-delete), new
  session (list-add), expand/collapse (arrow-right/arrow-down), jump to the end
  (go-down), the plan checklist (task-complete/box). A tool card also shows
  what kind of tool ran -- Theme.toolIcon(): pencil for patch/write, text page
  for read, folder for list/glob, magnifier for grep, braces for a shell,
  globe for web, list for a plan. Toolbar icons are 22 px (Breeze's toolbar
  size), card and row icons 14-16 px.

Colours (the "some text is unreadable" part)
- textFaint #6d757b -> #8d959c: ~3.4:1 -> ~5:1 on the card fill. It carries
  hints, provider URLs, session metadata and placeholders, so it is text, not
  decoration.
- A selected provider row and the open session row write on the accent fill;
  they now use accentText / accentTextDim / okOnAccent instead of faint grey
  and green on blue (named "accentText", not "onAccent": QML reads any `onFoo`
  property as a signal handler and refuses the assignment).
- The filled buttons (Send, Use this) take their ink from Theme.accentInk.

Composer
- The frame is one Theme.pad around its column on every side (anchors.fill,
  implicitHeight + 2*pad). With the top margin alone the last row -- the clip
  and Send -- hung over the lower border, which is what put the clip on the
  frame instead of inside it.

app/README.md gains an "Icons, colors" section (the provider, why the theme
search paths are filled by hand, the qtsvg requirement for rasterising Breeze,
the tool map, and the contrast rules).

Verified: --uitest 52/52, --selftest OK, and the 0.8.14 window relaunched in
the Plasma session (no QML warnings).

Version 0.8.14.

mode = coding

app/qml/ToolCard.qml

@@ -66,6 +66,14 @@ Rectangle {
                 radius: 3
                 color: card.stateColor
             }
+            /* What kind of tool this was, from the icon theme (Theme.toolIcon
+             * maps the engine's names onto Breeze ones). */
+            ThemeIcon {
+                Layout.alignment: Qt.AlignVCenter
+                name: Theme.toolIcon(card.name)
+                size: 16
+                color: Theme.textDim
+            }
             Text {
                 text: card.name
                 color: Theme.text
@@ -103,11 +111,11 @@ Rectangle {
                 implicitWidth: 26
                 implicitHeight: 22
                 visible: card.output.length > 0
-                contentItem: Text {
-                    text: card.expanded ? "▾" : "▸"
+                contentItem: ThemeIcon {
+                    name: card.expanded ? "arrow-down" : "arrow-right"
+                    glyph: card.expanded ? "▾" : "▸"
+                    size: 14
                     color: parent.hovered ? Theme.text : Theme.textDim
-                    font.pixelSize: Theme.smallSize
-                    horizontalAlignment: Text.AlignHCenter
                 }
                 background: Rectangle {
                     radius: Theme.radiusSm