Consolidate the duplicate src/context/ and src/contexts/ directories

The codebase has both a singular src/context/ (navigation-context, scroll-context) and a plural src/contexts/ (active-backend-context, conversation-websocket-context, settings-section-header-context, websocket-provider-wrapper). They hold the same kind of module, so contributors can't predict where a context lives, and imports reference both inconsistently (~36 files use #/context/).

This is a pure move plus an import-path update — no behavior change.

Framing correction (per review)

Both navigation-context and scroll-context should be relocated as-is, not folded into stores:

• NavigationContext is the seam that keeps components decoupled from react-router — important for the library/embedded use case and for tests. Folding it into a Zustand store would be worse than relocating it.
• ScrollContext is scoped to the chat DOM/provider tree and fits React context better than a global store.

Proposed approach

1. Move navigation-context.tsx and scroll-context.tsx into src/contexts/ as-is (no behavior change, not folded into stores).
2. Update imports across src, tests, and test-utils.tsx.
3. Remove src/context/ completely. (No re-export compat shim — pre-1.0, we don't keep compat shims.)
4. Optionally add a small lint/import guard or architecture test so #/context/ can't come back.

Acceptance criteria

• src/context/ removed; all contexts live under src/contexts/ (relocated as-is, not folded into stores)
• git grep -n "#/context/" src returns nothing
• npm run typecheck and the test suite stay green
• App boots cleanly in npm run dev

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This issue was drafted by an AI agent on behalf of the user, and refined per review feedback in the discussion below.