Speed Dial: replace tile monograms with native SVG icons

Tiles previously rendered a colored monogram (title/source initials plus an ordinal) with a per-id hue. Show the matching native SVG icon instead:

- AppAction/NativeCommand gain an `icon` field; commands get a curated key->icon table and settings inherit their group header's icon.
- Notebook tracks each page's resource icon name and reports it in tab_options(), so the tab picker can show it too.
- Searcher records the group icon so settings keep it through search.
- Web tile rendering swaps monogramFor/hue for an <img>; drop the now-unused hue/text CSS vars. Add a test asserting every non-empty icon resolves to a shipped SVG.

resources/web/dialog/css/theme.css

@@ -55,8 +55,6 @@
   /* Speed dial icon tile treatment */
   --speed-tile-bg: #efefef;
   --speed-tile-border: #d8d8d8;
-  --speed-tile-text-s: 74%;
-  --speed-tile-text-l: 34%;
 }
 
 :root[data-orca-theme="dark"] {
@@ -95,8 +93,6 @@
   /* Speed dial icon tile treatment */
   --speed-tile-bg: #34343b;
   --speed-tile-border: #50505a;
-  --speed-tile-text-s: 82%;
-  --speed-tile-text-l: 84%;
 }
 
 /* Re-theme the shared common.css chrome through variables (replaces dark.css's