fix(ui): keep the model picker from vanishing

[charlesvien]

Problem

Since #3821 the merged model/effort pill is the only model picker, and it hides itself whenever its thought-level option is missing. Two ways users lost it:

• Selecting Kimi K3 (no reasoning levels) removes the thought option, so the pill vanishes with no way to switch back. lastUsedModel persists the pick, so every composer restores Kimi and hides itself, surviving restarts. Kimi sessions lose the pill too because SessionView only mounts it when a thought option exists.
• A failed settings-store rehydration never sets _hasHydrated, so usePreviewConfig never fetches config and the pill never renders. Only clearing app storage fixed it.

Changes

• ReasoningLevelSelector renders a model-only pill for effort-less models (Reasoning submenu and slider hidden, menu opens on Advanced) and shows a disabled Loading placeholder during first load instead of nothing.
• SessionView mounts the pill when either the thought or model option exists.
• settingsStore.onRehydrateStorage handles the error argument (same pattern as pendingTaskPromptStore) so a failed rehydrate cannot wedge _hasHydrated.

How did you test this?

• New unit tests: effort-less model keeps the model picker and hides reasoning UI, stale effort label drops when switching to Kimi, loading placeholder renders, settings store marks hydrated on both success and failure.
• pnpm --filter @posthog/ui test (285 files, 2419 tests) and tsc --noEmit pass.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

[trunk-io]

😎 This pull request was merged.

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit 2fa37c4.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "mark settings hydrated after failed rehy..." | Re-trigger Greptile}

[stamphog]

--

[github-actions]

Contained UI bug fix (model picker visibility + rehydration flag) with new unit tests covering every stated scenario; no risky-territory surface touched, and the author has STRONG familiarity with these exact files.

• Author wrote 87% of the modified lines and has 25 merged PRs in these paths (familiarity STRONG).
• 👍 on the PR from greptile-apps[bot].

Gate mechanics and policy version

Gate		Result
prerequisites	✓	all clear
deny-list	✓	no deny categories matched
size	✓	138L, 3F substantive, 260L/5F incl. docs/generated/snapshots — within ceiling
tier	✓	T1-agent / T1c-medium (260L, 5F, single-area, fix)
stamphog 2.0.0b3		.stamphog/policy.yml @ d8556a9 · reviewed head 2fa37c4