The chat's persona is kept on the server, and nobody is slipped into a chat

[Khoality-dev]

Closes #334

What was wrong

• The server slipped a persona in. A new chat that named no persona was silently answered by assistants.default_persona_id. The desktop header said "Assistant" while Kurisu answered.
• The desktop's pick lived in local storage. Signing in again (the pick is cleared) opened an empty chat under "Assistant". Kurisu's conversation was in her bucket, and the desktop only looked in the assistant's.

What changed (the owner's decisions)

• Nothing is injected. A chat_request that names no persona is the assistant's own. pick_persona has no default step.
• The selection is saved on the server and loaded on open.
  • assistants.default_persona_id becomes selected_persona_id: who the chat is on. Clients read it when they open and write it when the user picks someone, and the server never applies it.
  • The migration 40309c742302 is a column rename (the FK constraint too), so each account opens on the persona it had.
  • Disabling a persona clears it, and deleting one clears it through the FK.
• Wire protocol 8. A removed field is a bump by the rules in version.py. Backend, constants.ts, the Android BuildConfig and protocol/events.json are all updated. The next release should be a new minor.
• Desktop:
  • personaStore loads the selection from GET /assistant and saves it with PATCH /assistant, instead of local storage. The old key is dropped with the other legacy keys.
  • The Personas page's Default badge and Make/Clear default become In the chat, Talk to and Use the assistant, all the same selection as the chat.
  • The assistant's update_assistant app tool takes selected_persona_id and goes through the store.
• Android:
  • The rename: the Personas page's tap selects, and the chat opens on the selection.
  • The header's Assistant row now works in a chat that has not started yet. It used to be held back because the server would have slipped the default in.
• Mock: follows the backend, including validation on PATCH /assistant.
• Docs updated throughout.

Tests

Each new test was run first against the unfixed code and failed:

• Backend: test_the_selected_persona_is_not_slipped_into_a_chat_that_names_none answered as Kurisu, and the TestTheChatSelection tests had no field.
• Desktop store: personaSelection.test.ts failed on load and on save.
• Android e2e: a_new_chat_can_be_the_assistants_own_while_a_default_persona_is_set found the row held back.

After the fix, all local:

• Backend 1366 passed; the one failure is the known local-only TestMime[notes.md].
• Mock 117/117. Workspace npm test: all suites pass. npm run typecheck: clean.
• Android JVM 276 tests. Android e2e MockBackendChatTest, NoPersonaTest and ConversationsTest: 7/7.
• Desktop Playwright: the full suite, 80 passed and 1 failed. The failure was the new sign-in-again spec, which wiped the server URL along with local storage. I fixed the spec, and personaOptional.spec.ts then passed 6/6.

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