fix(app): stop the Team model selector deadlocking on its first pick

[Rwanbt]

Problem

On a fresh Android install, every tap in the Team models picker raised
Échec de la configuration Team / Failed to save global Team configuration, and no model
could ever be checked. The Team selector was permanently unusable.

This is not the stale-sidecar / SPA-fallback bug fixed previously — verified: the
/team/config route is present both in source and in the embedded mobile CLI bundle.

Root cause

A cardinality contract that the client did not know about:

1. team-model-selector.tsx persisted every toggle. Guards existed for dropping below 2
   and exceeding 8, but none for the 0 → 1 ascent.
2. The server schema requires .min(2) (packages/opencode/src/team/selection.ts), enforced
   by validator("json", TeamSelection) on PUT /team/config → one model is rejected 400.
3. setSelected(next) ran after await save. The save always failed, so selected()
   stayed [] and the next tap replayed the same 400.

Reaching two models required passing through one, so it was structurally unreachable.

Desktop escaped it because the TUI dialog batches the whole selection behind a ready() gate
and only saves on confirm — users who configured there already had a valid stored selection
that onMount reloaded.

Fix

• New pure planTeamToggle() returning reject | stage | persist. Sub-minimal selections are
  staged locally and never sent; persistence happens on the toggle that reaches the minimum.
  Kept pure deliberately: packages/app has no testing-library, so an inline JSX guard would
  have shipped untested.
• MIN_TEAM_MODELS replaces the 2 hardcoded on both sides of the network boundary. The
  server schema remains the source of truth.
• Server error detail is now surfaced on Team and Debate save failures — a validation 400
  and a network failure previously produced the same opaque toast, which is exactly what made
  this bug unreadable on device.
• validateTeamSelection now requires two available models rather than all of them, and the
  persisted selection is filtered against connected providers on mount (mirrors
  DebateModelSelector). One retired model no longer invalidates a usable selection.
• The FAVORITES_GROUP sentinel used a raw NUL byte, which made git classify the file as
  binary — no diffs, no review. Replaced with the \0 escape: identical string value, file is
  textual again.

Same-class sweep

All six call sites of team.config / debate.config / .save() were inspected. Every other
one already gated correctly before persisting (dialog-team-setup.tsx batches behind
ready(), debate-model-selector.tsx returns early below one participant, submit.ts gates
on isValid). No other occurrence of "incremental persistence against a minimum-cardinality
schema".

Verification

• bun test --preload ./happydom.ts ./src → 695 pass / 0 fail (687 + 8 new).
• bun run typecheck → PASS. biome check → clean.
• Device test on a Mi 10 Pro (aarch64 release build, signed, installed over the previous
  build): first pick no longer errors, second pick persists, selection survives a restart, and
  removing below the minimum still shows the expected toast.

The 8 new tests pin the invariant directly: the first pick is staged and never sent, and
persistence only occurs at MIN_TEAM_MODELS.

Not included

The regenerated packages/mobile/**/opencode-cli.js bundles are left out. They are build
artifacts whose working-tree diff is dominated by CRLF churn and unrelated regeneration drift,
so they do not belong in this fix.

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