refactor(mecatui): surface abstraction for modals/overlays (keys + wheel + render + lifecycle in one place)

Problem

Adding a new modal/overlay to mecatui today touches seven places, all held in lockstep by convention:

1. a phase or Model flag (e.g. argsViewOpen),
2. a renderBody arm,
3. an onKey/dispatchPhaseKey branch (e.g. onApprovalKey's cascade),
4. an onMouseWheel branch,
5. an askButtonAt/hit-test arm in clickgeom.go,
6. lifecycle clears (advanceAsk, endRun, resetSession, retract),
7. the help overlay's key listing.

PR #553 (issue #488) drifted on two of these within one feature (the render path didn't thread the scroll offset; the idle-resume phase was wrong), which shows the convention is a weak gate. This came out of the code review discussion on #553.

Proposal

A surface interface so each modal/overlay owns its behavior in one file:

type surface interface {
    Render() string                                    // body content
    Regions() []ClickableRegion                        // emitted by the layout pass, not re-derived
    HandleKey(tea.KeyPressMsg) (Action, bool)          // keys the surface consumes
    HandleWheel(tea.MouseWheelMsg) (Action, bool)
    Close()                                            // lifecycle teardown
}

The Model holds a stack of active surfaces and routes render/keys/wheel/clicks to the top one — replacing the per-surface branches in renderBody, onKey, onMouseWheel, and the hit-test arms. Clickable regions carry a semantic Action payload (a sum type, not a func — value-safe across Model copies, testable), so the hit-test returns the action and ONE dispatcher executes it; no second mapping table.

Migration plan

• The click half (region-emitting builders + ClickableRegion) lands in PR #553 as the vocabulary/pattern proof.
• Migrate the approval surface first (it has the most arms: plan / args-view / generic modal), then the other overlays (MCP, team/agents, skills, soul, sessions, models, worktrees, schedule) one at a time.
• The seven-touchpoint list above becomes the acceptance checklist: a new surface should touch ONE file + ONE registration point.

Non-goals

• No widget hierarchy / retained-mode GUI framework — this stays Elm-architecture; surfaces are rebuilt from state every render (regions are emitted per layout pass, so they are always as fresh as the frame).
• No change to the wire/proto surface or the keymap registry semantics.

See also: PR #553 discussion (click geometry origin), ADR 0108.