refactor(ui/hooks): move pure derivations out of src/ui/hooks/

[lespaceman]

Files

• src/ui/hooks/useAppMode.ts (~24 LOC, confirmed pure)
• Likely siblings in src/ui/hooks/ — CLAUDE.md says ~30 hooks; expect 4-8 to be pure derivations

Problem

Files named useX imply React hooks (state, effects, refs). Pure if-else / derivation functions exported as useX for testability are fiction. The directory name lies: scanning src/ui/hooks/ to understand "what UI state exists" includes pure functions that hold no state. Locality of "what is React state?" is broken.

Sketch of deepening

1. Audit all of src/ui/hooks/ — identify any file with no useState, useEffect, useReducer, useRef, useContext, or hook composition.
2. Move pure derivations to src/ui/utils/ (or shared/utils/ if not React-flavoured) as named functions. Drop the use prefix.
3. Reserve src/ui/hooks/ for actual hooks.

Each move is a small mechanical refactor: rename, change import path at call sites, drop the unused dependency-array tracking if any.

Why this deepens

• Per-file leverage is small, but cumulative locality benefit when scanning the UI layer is meaningful.
• Removes a category of "is this a hook or just a function?" mental overhead.

Open design questions

• Is useAppMode really called from a single component? If so, inline rather than extract.
• Do any "pure" hooks actually use useMemo for memoization purposes? Those legitimately stay hooks.

Effort

Low per file. Total effort scales with how many of the 30 hooks turn out to be pure.

Risk

Very low — name + path changes only.