fix(icon): pin RuiIcon at --rui-icon-size in flex containers

[kelsos]

Summary

A flex item's default flex-shrink: 1 lets it compress below its declared width when a sibling exerts main-axis pressure — an explicit width doesn't prevent that. In RuiButton variant="list" the label is w-full text-nowrap, so a long label inside a width-bounded menu squeezes the RuiIcon SVG along the main axis while the height stays at the var-driven box, producing a sliver glyph (e.g. ~6×18 instead of 18×18).

RuiIcon's box is always intentionally driven by --rui-icon-size, so flex shrinking is never the desired behavior. Adding shrink-0 to the base classes pins every icon at its declared size for every consumer (list buttons, chips, toolbars) without per-call-site overrides.

Repro before the fix

<RuiMenu menu-class="max-w-[15rem]">
  <RuiButton variant="list">
    <template #prepend><RuiIcon name="lu-eye" /></template>
    Exclude events from Accounting
  </RuiButton>
</RuiMenu>

The eye icon renders at ~6 × 18px because the long label is w-full and pushes the SVG below its declared width.

Change

• RuiIcon.vue: prepend shrink-0 to the base iconStyles slot.
• RuiIcon.spec.ts: regression guard asserting shrink-0 is present so a future refactor can't silently drop it.

Test plan

• pnpm test -- --run RuiIcon — 1082 / 1082 pass
• Visual smoke in a long-label variant="list" menu (e.g. rotki's history-event action menu) — icon renders at 18×18 instead of being squeezed

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.80%. Comparing base (61b6d4c) to head (cfde3d0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #526   +/-   ##
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  Coverage   84.80%   84.80%           
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  Files         145      145           
  Lines        5260     5260           
  Branches     1571     1571           
=======================================
  Hits         4461     4461           
  Misses        799      799           

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