fix: remove stale WezTerm AppImage functions and update Julia to stable 1.12

[stanfish06]

Closes #95, closes #100

Changes

Remove stale WezTerm AppImage functions (fixes #100)

imgcat() and fzf-img() both hardcode a 2022 WezTerm AppImage filename that will silently fail on any session without that exact binary at ~/:

# removed
function imgcat() {
    ~/WezTerm-20221119-145034-49b9839f-Ubuntu18.04.AppImage imgcat "$1"
}
function fzf-img() {
    ...~/WezTerm-20221119-145034-49b9839f-Ubuntu18.04.AppImage imgcat...
}

Their kitty replacements already exist in the same file — alias icat="kitten icat" and fzf-kitty-img() — so the old functions are just dead code.

Update Julia from beta pin to stable 1.12 channel (fixes #95)

-juliaup default 1.12.0-beta1
-export PYTHON_JULIAPKG_EXE="$HOME/.julia/juliaup/julia-1.12.0-beta1+0.x64.linux.gnu/bin/julia"
+juliaup default 1.12
+export PYTHON_JULIAPKG_EXE="$HOME/.juliaup/bin/julia"

• juliaup default 1.12 tracks the stable 1.12.x channel and picks up patch releases automatically.
• $HOME/.juliaup/bin/julia is the stable juliaup shim — always resolves to whatever juliaup default is, so no version-specific path ever goes stale after a juliaup update.

Test plan

• juliaup status on HPC shows 1.12 as default
• julia --version returns a 1.12.x stable release
• kitten icat works as the image display alias
• fzf-kitty-img previews images correctly

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