[EuiPanel] Flat by default, softer corners, simpler padding

[ryankeairns]

Part of #9893

Depends on: #9894 (panel radius, quieter-edge decision, semantic spacing / spacing.panelInset)

Goal

Flat panels by default: no elevation shadow, visible quieter edge, panel radius, and a tighter preferred padding set.

Decisions (locked)

Defaults

• hasShadow → false (elevation opt-in)
• hasBorder → true for the default edged look
• Existing gates stay: border only for plain / transparent; elevation shadow remains incompatible with border (canHaveShadow requires !hasBorder)

Edge treatment (follows #9894)

• Today bordered panels use euiTheme.border.color (→ borderBaseSubdued)
• Quieter edge is not a Panel-specific color fork — it follows the Token updates: surfaces, radii, borders, semantic spacing #9894 / epic decision: quieter real border color or a shared faux border (box-shadow: 0 0 0 1px …) pattern
• Spike #9910 can land flat defaults + panel radius first; final edge recipe may adjust once that decision lands

Radius

• Apply border.radius.panel (12) — (today borderRadius="m" maps to border.radius.medium 4)

Related: other border.radius.panel consumers (design feedback)

Same panel-tier corner (12px) — can land with Panel or as a small follow-up PR once border.radius.panel exists (#9903):

• EuiCallOut
• EuiBanner
• EuiMarkdownEditor
• EuiModal
• EuiConfirmModal

Padding options (Panel only)

Reduce documented options; simplify decision making:

paddingSize	Role
none	Flush / nested / split panels
s	Dense UI
m	Default → spacing.panelInset / size.base
l	Roomier sections

• Soft-deprecate xs / xl on EuiPanel only (not a global PADDING_SIZES cull)
• Hard removal of xs/xl is not a blocker for the visual flip

Sequencing

Flat defaults + panel radius can land once border.radius.panel exists (#9903 / #9894). Edge quieting follows the #9894 faux-border vs real-border decision. Soft-deprecate padding in the same PR or a follow-up.

Scope

• Flip defaults: hasShadow={false}, hasBorder={true}
• Wire border.radius.panel
• Edge color / faux-border via shared tokens or pattern from Token updates: surfaces, radii, borders, semantic spacing #9894 (no one-off Panel paint)
• Map default / preferred padding to spacing.panelInset when the spacing alias exists
• Soft-deprecate Panel xs/xl; update docs to the preferred set

Related scope

• Wire CallOut / Banner / MarkdownEditor / Modal / ConfirmModal to border.radius.panel

Out of scope / later

• Hard removal of Panel xs/xl
• Global PADDING_SIZES collapse
• Form-control quieter edges — [HOLD][EuiFormControlLayout] Quieter borders #9897 (HOLD) / Token updates: surfaces, radii, borders, semantic spacing #9894
• Semantic spacing alias definitions themselves — Token updates: surfaces, radii, borders, semantic spacing #9894

[ryankeairns]

Design feedback: also wire EuiModal and EuiConfirmModal to border.radius.panel (12px).