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/SessionRow.qml

@@ -64,7 +64,7 @@ Rectangle {
                 text: srow.entry !== null && srow.entry.title.length ? srow.entry.title
                     : srow.entry !== null && srow.entry.firstUser.length ? srow.entry.firstUser
                     : "(no prompt yet)"
-                color: srow.open ? Theme.text : Theme.textDim
+                color: srow.open ? Theme.accentText : Theme.textDim
                 font.pixelSize: Theme.smallSize
             }
             Text {
@@ -78,7 +78,7 @@ Rectangle {
                     return n > 0 ? n + (n === 1 ? " message · " : " messages · ") + when
                                  : when
                 }
-                color: Theme.textFaint
+                color: srow.open ? Theme.accentTextDim : Theme.textFaint
                 font.pixelSize: Theme.smallSize
             }
         }
@@ -89,12 +89,12 @@ Rectangle {
             implicitHeight: 26
             ToolTip.visible: hovered
             ToolTip.text: "Delete this session and its transcript"
-            contentItem: Text {
-                text: "×"
-                color: parent.hovered ? Theme.err : Theme.textFaint
-                font.pixelSize: 15
-                horizontalAlignment: Text.AlignHCenter
-                verticalAlignment: Text.AlignVCenter
+            contentItem: ThemeIcon {
+                name: "edit-delete"
+                glyph: "×"
+                size: 14
+                color: parent.hovered ? Theme.err
+                     : srow.open ? Theme.accentTextDim : Theme.textFaint
             }
             background: Rectangle {
                 radius: Theme.radiusSm