Fix TUI crash when clicking past end of log line

[ryanmarshallmc]

Summary

• Fixes a panic in the TUI (railway dev) when clicking in the terminal beyond the end of a log line's actual characters
• The crash was caused by sel_start (derived from mouse click column) not being clamped to the line length, leading to an out-of-bounds slice index in render_log_line
• Adds .min(line_len) to clamp sel_start, matching the existing clamping already done for sel_end

Error before fix

thread 'main' panicked at src/controllers/develop/tui/ui.rs:230:41:
range end index 49 out of range for slice of length 12

Test plan

• cargo build passes
• cargo test passes

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