gui: Size the home window to its content

[BenWestgate]

Tails 7.13 field-test finding: the GUI home/welcome window should open at a compact natural height while longer workflow pages remain scrollable. Do not add Tails-version-specific geometry or special-case coordinates.

Implemented in one-file focused PR #78 on top of #77: remove the fixed 620px default height and retain the existing 880px width so GTK uses the home content's natural height. Exact-head Python-package run 466 is green.

The acceptance criterion is still partly visual. Keep this issue open through the final GUI rebase and confirm the compact home view plus normal scrolling at the supported Tails guest resolution before closing it.

[BenWestgate]

Release-gate status: #78 implements this as a one-file/one-behavior change at 38373b3, stacked on current #77, and is code-ACKed. It removes only the fixed 620px default height while retaining the 880px width and existing scrolling hierarchy. Remaining acceptance is manual: at the supported Tails guest resolution confirm the home opens at a compact natural height, then open a representative longer workflow page and confirm scrolling remains normal. Keep this issue open until that visual check is recorded.