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-plan/README.i18n.yaml

@@ -2,5 +2,5 @@
 # side as of the last confirmed-consistent state. Both languages carry equal authority;
 # after editing either side, bring the other along and re-record with:
 #   pnpm run verify-translation-pairing --write packages/client/ui-plan/README.md
-README.md: a8472c82ed1a5d6e5ebe4ae46b030ee140aa07c1
-README.zh.md: 77db21f515fd80d06719aec54170d451fbb67ad6
+README.md: de43ce66d17498d31e05f8c64092ea0843103054
+README.zh.md: b4d2f4fd1a6d45f814d4a20195434f34d207e9c8