fix: v0.3.4 – TimePicker value, dark-theme color-scheme

[Shewart]

Patch release for 0.3, porting two fixes and a test from #46.

• Package TimePicker threw when Value was set (580031c): it formatted a TimeSpan? with "HH:mm", a DateTime pattern, so any set value threw a FormatException. It now uses @"hh\:mm", like the CLI template.
• Dark theme didn't set color-scheme (b0b5a65): the browser drew its own controls (date pickers, scrollbars, select lists, autofill) light on a dark page. color-scheme: light / dark is now in the init theme, the blocks theme apply writes, the package's shellui-theme.css (and so shellui-all.css) and the demo.
• Value render test (58d26c7): renders every package component that takes a value with a sample value, and fails if one renders empty but throws once it has a value. It finds nothing else on 0.3.
• chore(release): v0.3.4 (a9da2d3): version bumped in the same 11 files as 0.3.3, plus a v0.3.4 section in docs/RELEASE_NOTES.md. The upgrade note tells existing projects to add the two color-scheme lines to input.css or rerun theme apply, since update doesn't rewrite input.css.

Checked:

• Release build has 0 warnings, and all 164 tests pass
• scripts/extract-release-notes.sh 0.3.4 extracts the new section
• The CSS bundle builds and contains color-scheme
• ShellUI.CLI and ShellUI.Components both pack as 0.3.4