Fix terminal origin jump when switching tabs

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Summary

Fixes #1513 — with multiple tabs open, the terminal content in different tabs could land at slightly different top-left origins, so switching between tabs produced a visible jump.

Reproduced on Arch Linux / Sway (Wayland) at display scale 1.0. The affected code paths in layout/mod.rs and screen/mod.rs are platform-independent, so the fix should apply equally across platforms.

The bug

ContextGrid::apply_taffy_layout positioned every panel using Taffy's layout_rect:

let [abs_x, abs_y, width, height] = item.layout_rect;
let x = (abs_x + self.scaled_margin.left) / scale;
let y = (abs_y + self.scaled_margin.top) / scale;

For a single-panel grid that rect isn't (0, 0) — Taffy gives the panel node a non-zero location based on the panel's own margin/padding, and the root container's gap. That offset then got added to scaled_margin on top of the window margin, so single-panel tabs ended up shifted away from the origin used when the first tab is set up in Screen::new (which writes renderer.margin directly to sugarloaf.set_position). The offset was small and only visible when flipping between tabs.

A second, narrower mismatch existed in Screen::create_tab: it fed current_grid().scaled_margin.left (physical pixels) into sugarloaf.set_position, which expects logical pixels. At scale 1.0 the two happen to agree, but on HiDPI displays the new tab landed at a different x than tab 1.

The fix

• layout/mod.rs: in apply_taffy_layout, when there's only one panel, bypass Taffy's layout_rect and use (0, 0) with the full available area minus the window margin. This is what actually resolves the cross-tab jump. Multi-panel grids still use Taffy's rect — they need it for split positioning and clipping.
• screen/mod.rs: create_tab now uses self.renderer.margin.left (logical margin), matching the position set for the first tab in Screen::new. This aligns the initial rich-text position with tab 1 on HiDPI.

Test plan

• With multiple tabs open (no splits): switch between them — the top-left of the terminal content stays fixed, no jump across tabs.
• With non-zero panel margin/padding in config: same scenario — origin still stays fixed across tabs.
• With a split pane and multiple tabs: switch between a single-panel tab and a split tab — both share the same top-left origin; split borders and clipping look unchanged.

Debugging and the proposed fix were generated with the assistance of Claude — please review critically.

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Fixed in main