UI: fix Bitcoin Core setup dialog sizing and copy #256

Problem

The hands-on Tails 7.13 review in #230 found two Bitcoin Core setup surfaces in install-core with layout/copy drift on the supported desktop:

• Further download verification is too narrow/tall and can clip signer/text content.
• The first-run Bitcoin Core Welcome/synchronization dialog is also oddly narrow/tall and its wording should match the actual launch order.

These are one install-core UI fix. Other UI regressions discovered in the same review now have their own canonical issues and PRs.

Goal

Make the Bitcoin Core install/setup dialogs fit the supported Tails desktop and state only what CipherStick actually does, without hard-coded geometry or a new UI framework.

Acceptance criteria

• The Further download verification prompt fits at the supported Tails guest resolution and identifies the selected Bitcoin Core version.
• Signer identity/fingerprint presentation is readable without clipping/ellipsizing important verification data.
• The first-run Bitcoin Core synchronization/welcome dialog fits without hard-coded window geometry.
• Synchronization wording matches the actual launch order.
• Pruned-storage explanation is concise and readable.
• Use supported existing Zenity/icon interfaces; add no dependency or branding asset.
• bash -n bails/.local/bin/install-core, ShellCheck, and git diff --check pass.
• Visually inspect the final dialogs on the supported Tails release at the standard guest resolution.

PR #262 is the implementation vehicle.

Separate scopes:

• Setup: replace post-install modal/countdown with concise completion #280 / setup: simplify completion UI #282 — post-install completion UI/countdown/terminal behavior.
• Chainstate preload: make status UI non-disruptive #281 / ui: quiet chainstate preload status #283 — chainstate preload status UI.
• Replace ibd-progress backup wizard with a Core RPC notification #265 / core: remove obsolete IBD backup watcher #286 — legacy ibd-progress watcher/wizard removal.

Refs #230, #249.