refactor: simplify toast reducer flow

[ALX99]

Simplification

Consolidate frontend toast creation in pi-web/src/web/state.ts so both explicit show_toast actions and failed server responses use the same toast() helper.

Why the current design is overcomplicated

The reducer had two implementations of the same responsibility:

• show_toast incremented toastSeq inline and constructed the Toast object directly.
• failed responses used the toast() helper, which also incremented toastSeq and constructed the same Toast shape.

That duplicate state-update path made toast sequencing and object construction slightly harder to audit than necessary. There is no behavior distinction between the two paths that justifies maintaining two implementations.

Behavioral contract

Behavior that must remain unchanged:

• show_toast creates a toast with a fresh numeric id, the requested text, and the requested kind.
• failed server responses create an error toast with the response error text.
• toast ids continue to be monotonically generated by the module-level toastSeq counter.
• dismiss_toast still clears only the currently visible toast when the id matches.
• All other reducer actions, message coercion, session/model state coercion, event handling, and exported types remain unchanged.
• No protocol shape, persisted data, configuration, public API, logging, or metrics changed.

Before and after

Before:

• show_toast duplicated the helper's logic by incrementing toastSeq and constructing { id, text, kind } inline.
• failed responses used toast(state, msg.error, "error").

After:

• show_toast calls toast(state, action.text, action.kind).
• failed responses continue to use the same helper.
• A focused reducer test covers both the explicit toast path and the failed-response path.

Specific evidence:

• Removed one duplicate toast construction path from pi-web/src/web/state.ts.
• Added pi-web/tests/state.test.ts to pin the shared toast behavior.
• Kept the reducer action type, state shape, helper behavior, and consumer-facing UI state unchanged.

Validation

Commands intended for this change:

• cd pi-web && npm run build
• cd pi-web && npm run typecheck
• cd pi-web && npm test

I could not run local commands in this automation environment because repository cloning/execution was unavailable. Static validation performed instead:

• Inspected pi-web/src/web/state.ts and confirmed show_toast now delegates to the existing toast() helper without changing action inputs or resulting state shape.
• Added pi-web/tests/state.test.ts covering explicit toast creation, matching-id dismissal, and failed-response toast creation.
• Confirmed pi-web/package.json runs node --test 'tests/*.test.ts', so the new test file is included by the existing test command.

Risk assessment

Behavior-sensitive areas examined:

• Toast id generation: still uses the same module-level toastSeq increment in the same helper already used by failed responses.
• Toast payload shape: still { id, text, kind }.
• Dismiss behavior: unchanged and now covered by the new test.
• Other reducer cases: left untouched.

The change is safe because it removes duplicated implementation while preserving the single existing helper's behavior.

Scope

Intentionally not simplified:

• The broader reducer structure, because command/event handling is behavior-dense and should remain easy to review.
• Message/session/model coercion helpers, because they sit on a wire-format trust boundary.
• WebSocket reconnect logic, because changing that would affect timing-sensitive behavior.
• subagents code, because existing open simplification PRs already touch that area.