fix(toast): raise sidebar toast placement

[ComputelessComputer]

Move the upgrade toast higher when the left sidebar timeline is active and cover the placement behavior with focused tests.

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Note

Low Risk
UI-only toast positioning and tests; no auth, data, or API changes.

Overview
When the left sidebar timeline is expanded, the main shell now passes placement="left-sidebar" to ToastArea instead of always using the default layout.

ToastArea gains an optional placement prop and moves toast positioning from the fixed top-14 class to explicit left / top styles. In left-sidebar mode it centers horizontally on the main chat surface ([data-chat-floating-anchor]) and sits slightly below its top edge, updating on resize, scroll, and ResizeObserver. Default mode keeps the prior top-timeline behavior (calc(50% + offset) and 56px top). Collapsed sidebar timeline still uses default placement.

New unit tests cover shell placement wiring and toast position behavior (default, anchored, scroll, and fallback when placement changes).

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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