refactor(shell): replace structural reducer with semantic SessionUiController

[lespaceman]

Files

• src/app/shell/sessionUiState.ts (~666 LOC, ~25-arm SessionUiAction discriminated union)
• All callers in src/app/shell/ and src/ui/ that dispatch({type: ...})

Problem

Public interface today is {state, dispatch} over a wide action union. Callers must learn the action shapes; tests poke the reducer with raw {type: 'TOGGLE_TODO_PANEL'} payloads. The interface is nearly as wide as the implementation — shallow.

Sketch of deepening

A SessionUiController whose interface is semantic verbs:

• scrollFeedUp() / scrollFeedDown()
• toggleTodoPanel()
• setInputMode(mode)
• focusMessage(id) / clearFocus()
• …etc, one method per action variant the UI legitimately needs

Reducer + action types become private. useReducer stays internally.

Why this deepens

• Interface shrinks (verbs replace 25 action constants).
• Test surface flips from "did action X mutate slice Y?" to "did intent Z produce the right user-visible state?" — the question that matters.
• Discoverability: a UI component author lists controller methods, not action types.

Open design questions

• Does the controller hold the state internally (useRef + reducer), or expose it via a hook (useSessionUi(): {state, controller})?
• Are any actions truly structural (e.g. used by replay/devtools)? Those would need an escape hatch.
• Migration: one big switch, or strangle by replacing call sites incrementally while both interfaces coexist?

Effort

Medium. ~25 action variants × call-site count. Mechanical once the controller shape is decided.