chore: enable TypeScript strict mode

[antfubot]

Why

The compiler ran with strict: false, so a large class of type errors (implicit any, nullable access, unsound casts) went uncaught. This turns on strict and resolves every error it surfaced, tightening the public type surface without changing runtime behaviour.

What

• Enable "strict": true in the root tsconfig.json.
• Migrate all 15 affected source files to satisfy strict checks: type previously-implicit any parameters, guard nullable DOM/instance access, make EventListeners.addTriggers generic over the trigger union, narrow ComputePositionConfig, and add PropType/slot-data types to the components and directives.
• Export the directive/slot types that now appear in emitted declarations so .d.ts generation stays valid.

Verification

• pnpm typecheck (root tsc) and pnpm -C packages/floating-vue typecheck (vue-tsc) both pass.
• pnpm test — build succeeds, 31/31 tests pass.
• pnpm lint passes.

This PR was created with the help of an agent.

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