Update figures and captions for reviewer comments

[kathsherratt]

Summary

• R1: Improve Table 1 clarity #117 (Table 1): Added "Numbers in brackets denote standard deviations" to Table 1 caption
• R1: Revise Figure 1 to avoid misleading messages #118 (Figure 1): Changed panel A palette to Set2, panel B to teal/orange — clearly distinct color schemes. Added "unadjusted means" note to caption with reference to Figure 2 for adjusted estimates
• R2: Fix Figure 3 description and legend #125 (Figure 3): Added scale_shape_manual() with drop = FALSE so both Single-country and Multi-country always appear in legend. Updated caption to describe colour (method), shape (country targets), and line style (adjusted/unadjusted)
• Journal: Upload figures in .tif or .eps format #135 (TIF export): Added commented-out ggsave() block for TIF export of all 3 figures at 300 DPI
• Also carries forward PR 1 fixes (format: html, epi_target column rename shim) so the branch renders

Test plan

• quarto render report/results.qmd renders without errors (verified locally)
• Visually inspect Figure 1 panels A and B use distinct palettes
• Visually inspect Figure 3 legend shows both Single-country and Multi-country shapes
• Table 1 caption includes SD explanation

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