[Feature]: Mobile: add the Legacy features menu (Plan mode toggle) for parity with web/desktop

[amanthanvi]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/mobile

Problem or use case

On web/desktop, Settings → General has a "Legacy features" section (LegacyFeaturesSection, apps/web/src/components/settings/SettingsPanels.tsx) with three toggles:

• Plan mode (legacy) — planModeEnabled (ClientSettings): restores the Build/Plan composer toggle, the /plan and /default commands, and Shift+Tab.
• Stream token by token (legacy) — enableLegacyTokenStreaming (ServerSettings).
• Sidebar (legacy) — legacySidebarEnabled (ClientSettings).

The mobile app has no equivalent menu. Its Settings screen has a one-row "Legacy" section containing only "Legacy Thread List" (device-local legacyThreadListEnabled, apps/mobile/src/features/settings/SettingsRouteScreen.tsx). Meanwhile, plan mode on mobile is in a strange in-between state:

• /plan and /default are always present in the composer's slash-command menu (apps/mobile/src/features/threads/ThreadComposer.tsx) — there is no way to hide them, and no toggle that corresponds to web's planModeEnabled.
• There is no Build/Plan mode picker UI at all, so a thread that was put in plan mode from another client is hard to notice and manage from the phone.
• Mobile does not consume ClientSettings at all (apps/mobile/src/persistence/mobile-preferences.ts notes "Mobile has no client-settings sync"), so a user who enables Plan mode (legacy) on desktop gets inconsistent behavior when they pick up their phone.

Concretely: I use Plan mode (legacy) on desktop. When I open the same environment from mobile there is no Legacy features menu, no mode toggle, and no way to control any of this — the two clients disagree about which legacy features exist.

Proposed solution

Add a Legacy features section to mobile Settings mirroring the web/desktop one, scoped to what makes sense on mobile:

1. Plan mode (legacy) toggle, reusing the existing shared planModeEnabled key from packages/contracts/src/settings.ts (ClientSettingsSchema / ClientSettingsPatch) so the preference syncs with web/desktop rather than adding another device-local flag. When on, surface a Chat/Plan mode control in the mobile composer (parity with web's "Mode" radio with "Chat" and "Plan" items in CompactComposerControlsMenu.tsx); when off, hide /plan and /default from the slash menu and force default mode, matching web's gating in ChatView.tsx.
2. Fold the existing Legacy Thread List row into the same section so mobile has one coherent "Legacy features" menu like desktop.

(Stream token by token (legacy) is server-authoritative and mobile has no server-settings editor, so it is reasonably out of scope.)

Why this matters

Cross-client consistency: legacy features are already a first-class, discoverable concept on web/desktop, and the settings keys already live in shared contracts. Mobile users who rely on plan mode currently get an undiscoverable, unconfigurable subset — and users who don't want plan mode can't turn the slash commands off, unlike on desktop.

Smallest useful scope

A "Legacy features" section on the mobile Settings screen with a single Plan mode (legacy) toggle backed by the shared planModeEnabled client setting, gating the /plan / /default slash commands the way web does. Everything else (mode picker UI, folding in Legacy Thread List) can come later.

Alternatives considered

• A device-local mobile-only flag (like legacyThreadListEnabled): works, but adds a third divergence instead of reusing the contract key that web/desktop already share.
• Leaving plan mode always-on via slash commands: that is today's behavior, and it is both undiscoverable and inconsistent with the desktop toggle semantics.

Risks or tradeoffs

• Mobile currently has no ClientSettings sync at all, so the first shared-setting consumer carries the plumbing cost (or falls back to the device-local pattern with the sync noted as a follow-up).
• Threads already in plan mode when the toggle is off need the same handling web has (force default mode).

Examples or references

Web/desktop implementation for parity reference:

• apps/web/src/components/settings/SettingsPanels.tsx — LegacyFeaturesSection (section header "Legacy features")
• apps/web/src/components/settings/settingsSearch.ts — legacy-plan-mode, legacy-token-streaming, legacy-sidebar
• packages/contracts/src/settings.ts — planModeEnabled, legacySidebarEnabled, enableLegacyTokenStreaming
• apps/mobile/src/features/settings/SettingsRouteScreen.tsx — mobile's current one-row "Legacy" section

Screenshots (captured on current main @ ac1264e2c; web via local dev server, mobile via the iOS dev client com.t3tools.t3code.dev built from source on an iOS 26.5 simulator):

Web/desktop — Settings → General → Legacy features (the parity target):

Web/desktop — composer Mode control (Chat/Plan) with Plan mode (legacy) enabled:

Mobile — Settings has only a one-row "Legacy" section (no Legacy features menu, no Plan mode toggle):

Mobile — /plan and /default are always in the slash menu, with no setting to control them:

Contribution

• I would be open to helping implement this.