Fix plotSpectra axis limits

[gustavdelius]

Summary

Fixes #340.

• plotSpectra() was pinning the y-axis lower limit to 1e-20 even when ylim[1] was not specified, causing a large empty region below the data on the log-scale plot. The 1e-20 threshold is now used only to filter near-zero values from the data; the y-axis auto-scales to the data range when ylim[1] is NA.
• The default lower x-axis limit (wlim[1]) now depends on the resource argument: min(params@w) / 100 when resource = TRUE (existing behaviour, to show some resource below the fish grid) and min(params@w) when resource = FALSE.
• Documentation for wlim and ylim parameters updated to describe this behaviour.

Test plan

• Existing axis-limit test updated: y-axis lower limit is now NA (auto-scale) when ylim[1] is not specified.
• New test verifies that default wlim[1] extends below min(params@w) with resource = TRUE and equals min(params@w) with resource = FALSE.
• Full plot test suite passes: testthat::test_file("tests/testthat/test-plots.R") — 72 tests, 0 failures.

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