feat: Upgrade to React 19 rc

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.83%. Comparing base (5a060a9) to head (51ecf09).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2565 +/- ## ========================================== + Coverage 90.74% 90.83% +0.09% ========================================== Files 66 66 Lines 2646 2762 +116 Branches 526 526 ========================================== + Hits 2401 2509 +108 - Misses 77 79 +2 - Partials 168 174 +6 ``` | [Flag](https://app.codecov.io/gh/vaadin/hilla/pull/2565/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/vaadin/hilla/pull/2565/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin) | `90.83% <ø> (+0.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vaadin#carryforward-flags-in-the-pull-request-comment) to find out more.

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[sonarcloud[bot]]

Quality Gate failed

Failed conditions
C Reliability Rating on New Code (required ≥ A)

See analysis details on SonarCloud

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[Artur-]

This cannot work until preact signals supports React 19 because the symbol name used for react elements has changed, see https://github.com/preactjs/signals/issues/580#issuecomment-2196321246