feat: cmd+shift+delete deletes the highlighted conversation

The delete action in the chat view had no shortcut, so removing a
conversation meant opening the action panel. It now takes cmd+shift+delete,
and Chat Sessions uses the same combination so one shortcut means one
thing across both commands (it was cmd+delete there).

The key macOS labels "delete" is backspace, and Vicinae's key map has
`"backspace" -> Qt::Key_Backspace`, rendered as the delete token. That
mapping is why the shortcut string is "backspace" rather than "delete":
`"delete"` maps to Qt::Key_Delete, which is the forward-delete key, and
`"deleteForward"` aliases backspace too.

Not verified by running it. I tried to confirm the shortcut renders by
temporarily promoting the action to primary so the bottom bar would show
it, but an A/B test with cmd+shift+c proved the action bar only ever
renders the primary action's implicit Return, never a custom shortcut:
both rendered as a lone return glyph. Firing a shortcut needs a real
keystroke, and this machine cannot send one. The reasoning above is from
Vicinae's keyboard.cpp key map, not from observation.

typecheck, eslint, 156 tests and vici build all pass.

README.md

@@ -57,7 +57,7 @@ To change the model later, either edit the **Model** preference directly, or run
 | `↩` | Send the message in the search bar |
 | `↩` with an empty bar | Open the highlighted conversation |
 | `⌘ N` | New chat |
-| `⌘ ⌫` | Delete a conversation (in **Chat Sessions**) |
+| `⌘ ⇧ ⌫` | Delete the highlighted conversation (also in **Chat Sessions**) |
 
 Prompts are single line. An earlier `⌘ ↩` multi-line composer was removed — see [Architecture](#architecture) for why a `Form.TextArea` cannot be laid out cleanly in a launcher. The `Tab` shortcut went with it: the left column *is* the conversation switcher now, so an accessory dropdown would only duplicate it.