fix: failed diagram modal size and table viewer readability (P3-20, P3-21)

[dvzrd]

Summary

Dogfood fixes after #134.

• Failed diagram modal: a failed graphics render no longer asks for the whole screen. ImageViewer::want sizes the panel to the error line plus the source, and scroll_text is bounded by the displayed lines.
• Table viewer: columns use their natural width with horizontal scroll (no 40-column cut), the last column fills the window with no dangling │, and a ─┼─ rule sits under the header.

Test plan

• ./scripts/check.sh (default and lite) green on the top of the stack, whose tree matches this branch plus the later PRs.
• New tests: failed-render size, filled last column, a 60-character cell shown in full, header rule ┼.
• Operator: Mermaid bad opens small; table viewer on wiki/architecture/prior-art-and-libs.md and the roadmap tables.

Stack: this PR → #136 (status enum) → #137 (toolbar icons) → #138 (code viewer). Merge in order; each later PR is rebased onto main after the one below it is squash-merged.

🤖 Generated with Claude Code

[dvzrd]

Landed on main as part of #138 (squash commit aab5f32). #138 was retargeted to main by mistake while merging the stack in order, so its squash carried this PR's commits too. main's tree is identical to the stack tip that ./scripts/check.sh and CI passed on, so nothing is missing. Closing as merged-via-#138.