Fix: double-click + backspace clears only one char in text inputs

[StephaneDelcroix]

Bug

When creating a new session, double-clicking the session name entry selects the full name, but pressing backspace only clears one character instead of the full selection. Same issue in rename inputs.

Root Cause

@onkeydown on value-bound inputs triggers a Blazor re-render before the browser processes the keystroke. The re-render writes back element.value, which clears the browser's text selection. When backspace then fires, there's no selection to delete — only one character is removed.

Fix

Change @onkeydown to @onkeyup on all value-bound text inputs. @onkeyup fires after the browser's default action, so the selection is preserved and multi-character delete works correctly.

Affected inputs:

• CreateSessionForm.razor: session name, worktree branch, worktree PR
• SessionListItem.razor: rename input
• SessionCard.razor: rename input

Non-value-bound inputs (Dashboard/SessionSidebar group name inputs) are unaffected and left unchanged.

Tests

Added InputSelectionTests.cs with:

• Convention test scanning all .razor files for value-bound inputs with @onkeydown (prevents regression)
• Specific tests verifying each fixed input uses @onkeyup