[marcinz606]

UI polish for the right panel. Each change is a separate commit.

Changes

• Lighter surfaces. Every dark surface, border and warm selection chip moves up by the same step. The panel changes from #0D0D0D to #252525. Text, accents, channel colors and photo marks keep their values. Hint text is 5.0:1 on the panel.
• Card outline. A card header inherited QPushButton's rounded bottom corners and had no side outline. The seam line curled up at both ends, and the body's outline started with its own corner. Now the header has an outline on all sides and square bottom corners. A collapsed card keeps its round corners ([checkable="true"]:!checked, because Qt matches nothing on :checkable or :unchecked).
• Tab strips on a track. The main tab strip was a full-width band in the card header color, so it merged with the first card's header. The page tabs sat on the bare panel. Now every OverflowBar draws the SegmentedChoice track, PanelTab does not draw its hairline, and the main strip has a space_sm gap above the cards.
• Readable error hints. QLabel[hint="error"] used error at the small size, 2.9:1 on a card. A new error_text tier (#FF5A5A, 4.7:1 on a card) gives error copy the same text tier that the channel colors have. Error hints now show at body size. error stays for invalid-input edges and the failed-frame badge, which has white on it.
• Find field lined up. The Find field ran 8px in from each side. Now it uses the same edge as the tab strip and the cards. Its top and bottom margins still drag the panel.
• Hint icons. Before, a warning hint differed from a plain hint only by its amber color. Now hint_label gives a warning or an error hint an alert icon in its own color, in a left margin, so the hint does not depend on color alone.
• Slider focus. The 2px white ring on the slider handle is gone. A Tab-focused slider row now gets the same border_focus edge as a focused button. A ring on the handle cannot have a gap and still fit in the 15px track. A click or a drag shows no edge.
• Softer focus edge. border_focus changes from #949494 to #7A7A7A. It is 3.6:1 on the panel and 3.3:1 on a card.

Notes

• Only the UI changes. Rendered photos and thumbnails do not change, so THUMBNAIL_RENDER_VERSION is not bumped.
• The guided tour uses border_focus for its highlight ring, so the ring is also softer.
• Amber warning hints stay at the small size. Most of them are gray hints that turn amber, so a size change would make the layout jump.

Testing

• Rendered the real app headless under Xvfb at 2× for each change: Frame, Look and Export tabs, open and collapsed cards, a focused button and the soft-proof error hint.
• make lint and make type pass.
• Tests for the changed tokens and widgets pass: theme tokens, toggle look, panel tab, overflow bar, collapsible, right panel, sidebars and tutorial overlay.