Skip echoing client-originated values via bool flag on ValueElement

[evnchn]

Motivation

When a ValueElement (e.g., ui.input, ui.select) receives a value change from the client via LOOPBACK, the server currently echoes the same value back in the next outbox update. This is redundant — the client already has the value — and wastes bandwidth, especially for elements where the value is the main part of the payload.

This implements the approach proposed by @falkoschindler in #5728: a single boolean flag on ValueElement that tells the outbox to omit VALUE_PROP when the value originated from the client.

Supersedes #5728 with a much simpler implementation (no diff machinery, no deepcopy, no new merge semantics).

Implementation

• ValueElement._value_from_client flag, set to True when the LOOPBACK handler processes a client-originated value change
• Outbox._build_element_dict() checks the flag via duck-typing (getattr); if set, strips VALUE_PROP from the props dict before sending
• nicegui.js merges incoming props with existing element props ({ ...existing.props, ...element.props }) so that an omitted VALUE_PROP preserves the client's current value rather than blanking it
• Reconnection safety: _to_dict() is unchanged and always returns full state — build_response is not affected by the flag
• Flag reset: the flag resets to False on any server-originated value change (_handle_value_change with _send_update_on_value_change=True), ensuring server-set values are always sent

Progress

• The PR title is a short phrase starting with a verb like "Add ...", "Fix ...", "Update ...", "Remove ...", etc.
• The implementation is complete.
• This PR does not address a security issue.
• Pytests have been added/updated.
• Documentation has been added/updated or is not necessary.
• No breaking changes to the public API or migration steps are described above.