chore: migrate templates from .tile shim to Bulma 1.x .columns

[jmeridth]

Closes #66

What

Replace the deprecated `.tile` markup in `_layouts/default.html`, `_layouts/aggregate_default.html`, `_layouts/author_default.html`, and `_includes/authors.html` with Bulma 1.x `.columns` / `.column`. The outer `tile is-ancestor is-vertical` wrapper becomes a plain block container; the `tile is-parent` row becomes `

`; the 8/4 children become `

` and `

`; the `tile is-parent is-vertical sidebar` becomes a plain `

`; and the `tile is-child widget` widget wrappers become plain `

`. Delete the 46-line tile-system shim block from `assets/css/custom.css`.

Why

PR #65 vendored Bulma 1.0.4, which removed the `.tile` layout primitives entirely. To preserve the existing layouts without rewriting templates in that PR, a temporary CSS shim was added to `custom.css`. This PR completes the migration so templates use Bulma's actual grid, and the shim block is no longer needed in the repo.

Notes

• The `.tile.is-ancestor.is-vertical` outer wrapper had a single purpose: vertically stacking the nav above the content row. `
  ` elements stack by default in normal flow, so the wrapper has been removed entirely rather than translated to `.columns.is-vertical` — that exists but adds complexity for no reason.
• `.sidebar` and `.widget` are kept as plain div selectors so existing CSS hooks (e.g. `.sidebar { padding: 0 0.75rem 0.75rem 0.75rem !important; }` in custom.css) continue to apply unchanged.
• Bulma's `.columns` collapses to a single column at mobile widths (< 769px) and goes side-by-side at tablet+, matching the tile system's responsive behavior exactly. No additional `is-mobile` / `is-desktop` modifiers needed.
• `_layouts/aggregate_default.html` and `_layouts/author_default.html` still inline their own nav copy rather than using `_includes/nav.html`. This pre-existing duplication is unchanged.

Testing

Verified locally with cache wiped (`rm -rf _site .jekyll-cache`) and `bundle exec jekyll serve --livereload --future`:

• Home page renders with sidebar (Recent Posts, Authors) on the right at desktop widths, stacked below at mobile widths.
• Any author page (`/_chrismissal/`, `/_jimmybogard/`, etc.) renders with sidebar (Recent Author Posts, Recent Site Posts, Authors) on the right at desktop widths, stacked below at mobile widths.
• Any aggregate post page renders with the same sidebar pattern.
• Navbar (with theme toggle) still renders correctly across all three layouts.
• Theme toggle still cycles light → dark → system with the right visual result on the new column layout.
• Burger menu still toggles correctly at mobile widths.
• `grep -rn '\btile\b' _layouts _includes assets/css` returns zero matches.