feat(web): replace the plan select with a status chip and placeholder swap

Plan mode is entered through /plan only — the select control is retired.
The conversation.input.plan seat (now right of the access-mode control)
renders a read-only Plan chip while the projection's effective target is
plan mode; its hover x executes /plan off, and the chip follows the folded
target (appears on /plan immediately, disappears on /plan off) with frames
correcting either way. While plan mode is targeted the composer textarea's
placeholder switches to the plan-task wording — InputBar reads the same
projection through the standard-kit useProjection (the TodoDock posture:
a type-only key merge, no domain service edge), and owner placeholders
still win.

packages/client/ui-conversation/tsconfig.json

@@ -26,6 +26,9 @@
     {
       "path": "../../session-projection/session-projection"
     },
+    {
+      "path": "../../plan/plan-mode"
+    },
     {
       "path": "../../todo/tool-todo"
     },