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/ThemeIcon.qml

@@ -0,0 +1,50 @@
+import QtQuick
+
+// One icon, from the desktop's icon theme (image://csttheme/, see
+// app/src/ui/ThemeIcons.cpp), tinted with a color of the window's own.
+//
+// `glyph` is the last resort: a machine with no icon theme at all gets the
+// unicode character the app used before, rather than an empty hole. Every
+// place that shows an icon names its glyph, so nothing can go blank.
+Item {
+    id: root
+    property string name: ""
+    property string glyph: ""
+    property color color: Theme.textDim
+    property int size: 16
+    implicitWidth: size
+    implicitHeight: size
+
+    /* "#rrggbb": a color typed to a string in QML carries alpha and a
+     * leading '#', which the provider's query would choke on. */
+    function hex(c) {
+        function two(v) {
+            const s = Math.round(Math.max(0, Math.min(1, v)) * 255).toString(16)
+            return s.length < 2 ? "0" + s : s
+        }
+        return "#" + two(c.r) + two(c.g) + two(c.b)
+    }
+
+    Image {
+        id: img
+        anchors.fill: parent
+        source: root.name.length
+                ? "image://csttheme/" + root.name + "?color=" + encodeURIComponent(root.hex(root.color))
+                : ""
+        sourceSize.width: root.size
+        sourceSize.height: root.size
+        fillMode: Image.PreserveAspectFit
+        smooth: true
+        visible: status === Image.Ready
+    }
+
+    Text {
+        anchors.fill: parent
+        visible: img.status === Image.Error && root.glyph.length > 0
+        text: root.glyph
+        color: root.color
+        font.pixelSize: Math.round(root.size * 0.85)
+        horizontalAlignment: Text.AlignHCenter
+        verticalAlignment: Text.AlignVCenter
+    }
+}