fix(app): stabilize server routes and selects

[opencode-agent]

Summary

• parse server-keyed routes without throwing and redirect malformed URLs to Home
• retain the last valid server key while Solid Router tears down a matched route
• derive nested session actions from the mounted server provider instead of mutable route params
• defer controlled Select/SelectV2 callbacks until Kobalte finishes closing and cleaning up its portal

Closes #39415.
Closes #39455.

Checks

• bun test src/utils/session-route.test.ts src/context/layout.test.ts (packages/app)
• bun typecheck (packages/app)
• bun run typecheck:e2e (packages/app)
• bun test src --only-failures (packages/ui)
• bun typecheck (packages/ui)
• Prettier check on changed files
• Oxlint on changed files (0 errors; existing warnings only)

Full app unit suite: 691 passed; the existing i18n parity test failed because Arabic is missing five unrelated pre-existing keys.

Requested by: @Brendonovich (Brendan via Slack)