[UI] Stop the stack preview overrunning a narrow terminal

[KCaverly]

The split sized its two columns with independent clamps, so each could
satisfy its own floor while their sum overran the terminal. At 30 columns it
drew 37-column rows — every row wrapped, shearing the layout — and there was
no width below which it declined to open at all.

rightW is now derived from leftW rather than clamped separately, which is
what makes the three parts sum to exactly m.width by construction instead of
by arithmetic that happened to work at common sizes.

Below 60 columns the two columns plus the divider cannot both carry useful
content, so the screen falls back to the plain stack list. That covers the
resize-down case for free: the split flag stays set and the preview returns
when the terminal is widened again, with no keypress. splitShown is the one
predicate the renderer, the key routing and the wheel all consult, so they
cannot disagree about whether there is a pane on screen — keys driving an
invisible pane would be the natural next bug here. Pressing d while too
narrow flashes rather than silently doing nothing.

The sweep test walks every width from minViableWidth to 200 and fails on any
row wider than its terminal; it reproduces the original 37-in-24 break.

Co-Authored-By: Claude Opus 4.8 noreply@anthropic.com

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Stack (bottom → top):

1. [UI] Drop the branch-diff jump from the stack screen #45
2. [UI] Stop the stack preview overrunning a narrow terminal #46 ← this PR
3. [Git] Pin the diff flags the body parser depends on #47
4. [UI] Teach both diff viewers the same file-jump key #48
5. [Diff] Thread the [diff] config through the viewer and zoom with z #49
6. [Git] Carry diff args and excludes through every diff call #50
7. [Diff] Add the external diff pager package #51
8. [Config] Accept a [diff] section with an optional pager #52
9. [Docs] Document the diff viewer settings #53