ui: fix Welcome and chainstate dialog visuals

[BenWestgate]

Two small zenity fixes seen while testing a fresh install on Tails.

Welcome dialog shape. The first-install Welcome dialog in install-core set no width, so zenity wrapped its paragraphs into a tall, narrow column. It now passes --width=600, which Tails' zenity honours. The dialog's text is unchanged: it keeps the paragraphs adapted from Bitcoin Core's own first-run dialog. (An intermediate commit shortened them; it is reverted here, since CipherStick users run the same Bitcoin Core and should see the same explanation.)

Chainstate dialog icon. The "Chainstate preloaded to RAM" and "Your computer is low on memory" dialogs in chainstate-preload used drive-harddisk-solidstate, which suggests disk rather than memory. Adwaita has no RAM icon, so both now use the stock computer icon: the chainstate moves off the USB stick into the computer's memory. No new icon files are added.

Testing: --width confirmed on Tails. shellcheck with CI's flags reports nothing new.

🤖 Generated with Claude Code

https://claude.ai/code/session_01LvEHMDeHvYQYDEkpJn95X8

[BenWestgate]

github-advanced-security is red here, but not because of this change. The check fails on every open PR in this repo, including ones that don't touch any of these files, and the failing run has no title, summary or findings. It comes from a repository setting, and I've been told to ignore it. No fix exists for it in this PR's scope. All the other checks pass: CodeQL, Analyze (actions), Check shell code and dependency-review.

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[BenWestgate]

@codex review

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[BenWestgate]

Codex current-head review at 91b927b: no findings. The diff is limited to a 600px Welcome-dialog width hint and replacing the misleading storage icon with the stock computer icon; neither changes setup control flow.

[BenWestgate]

Rebased onto master and squashed into one commit (9b4efd3) to fix the conflict with #303 in chainstate-preload. The diff is the same as before: two icon changes and --width=600 on the Welcome dialog. If #283 lands first, it deletes the "Chainstate preloaded" dialog, and then only the low-memory icon change from this PR remains.

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Generated by Claude Code

[BenWestgate]

AI-assisted current-head review: ACK 9b4efd3. The Welcome width and stock notification icons change presentation only. No code findings; confirm the dialogs on supported Tails.