wire builtin.editor to the active buffer + open files from the tree

EditorController mirrors the daemon's active buffer into the Flutter
UI. Hydrates on mount (editor.active → editor.read), subscribes to
editor.opened / active-changed / edited / saved / closed, and pushes
local edits back via editor.set-content. A one-shot echo-suppression
flag keeps the daemon's authoritative edit echo from clobbering the
caret the user just moved.

Text surface is EditableText directly — no TextField / Material
import, so D-007's "bare WidgetsApp" guardrail carries into the
editor tab. JetBrainsMono via the shared mono-family constants;
cursor + selection colours come from the theme tokens.

File-tree click now calls ipc.request('editor.open', {path}) instead
of the kernel command registry (which wouldn't have reached the
daemon). The opened file becomes active, the editor tab's controller
sees editor.active-changed, and the text content reconciles.

Linux release bundle still builds; 174 app tests + 74 core tests
stay green.

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

app/lib/kernel/src/i18n/catalog/builtin.editor_en_us.json

@@ -0,0 +1,5 @@
+{
+  "tab.title": { "translation": "Editor" },
+  "empty": { "translation": "Open a file to begin editing." },
+  "subtitle.no-buffer": { "translation": "no buffer · use `clide open <path>` or pick a file in the tree" }
+}