The tab row: a thinner line, a fainter glow, a step closer to the edge

[WaylandYang]

Follow-ups to #423, three small ones on the tab row:

• The active tab's underline is 1 px, not 2 — the glow already says which tab is active, so the line only has to mark the edge. Inactive tabs carry the same transparent line, so nothing shifts.
• The glow is fainter: white 6 % at the bottom edge fading out by half the tab's height (was 10 % to 60 %).
• The row moves a step toward the edge: the nav's inset drops from 16 to 8 px, so the first tab's icon sits 24 px in — the tabs carry 16 px of their own padding, and at the same inset as the wordmark the row read as indented further than it.

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