feat(notify): Windows toasts under talk's own identity, icon on Linux

The daemon (and talk test --toast) now register the AppUserModelID
m3ta.talk under HKCU with display name and the installed icon, so
toasts appear as "talk" with the icon instead of Windows PowerShell;
the PowerShell identity stays as fallback until the registration has
run once. The redundant "talk" title line is dropped under the own
identity. notify.Setup bundles icon install, identity and the talk:
protocol handler for both callers. Linux notifications carry the
installed icon as app_icon. doctor names the active identity.

Verified on the Windows 11 VM: registry key, icon file, toast with
working Cancel button, exe icon; Linux via quickshell.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/spec.md

@@ -131,3 +131,9 @@ These points replace the corresponding statements above (grilling state of
 - **Notifications without beeep:** desktop toast via the OS's own tools
   (notify-send / osascript / PowerShell), exec-based like osbuiltin;
   initially only for `quiet_mode = "toast"`. Cancel button: later.
+- **Icon and toast identity (2026-10-25):** one SVG icon (speech bubble
+  with level bars), rendered into the binary: PNG for notifications (Linux
+  app_icon, Windows toast), `.syso` resources for talk.exe. On Windows the
+  daemon registers talk's own AppUserModelID `m3ta.talk` under HKCU (no
+  shortcut, no admin), so toasts appear as "talk" with the icon instead of
+  Windows PowerShell.