Enable the clippy::pedantic lint group (#8429)

Instead of opting in to pedantic lints one by one, enable the whole
group and opt out of the noisy ones.

64% of the pedantic lints were already listed individually. This deletes
90 explicit lint lines, enables 51 pedantic lints we never listed, and
picks up new pedantic lints for free. Each opt-out carries its hit
count, so the cost of turning one back on is visible.

`restriction` and `nursery` stay opt-in per lint.

Stacked on top of #8430, which fixes the one real bug the new lints
found.

* [x] I have followed the instructions in the PR template

---------

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/egui_extras/src/syntax_highlighting.rs

@@ -210,6 +210,13 @@ impl SyntectTheme {
     derive(serde::Deserialize, serde::Serialize),
     serde(default)
 )]
+#[cfg_attr(
+    all(feature = "serde", not(feature = "syntect")),
+    expect(
+        clippy::unsafe_derive_deserialize,
+        reason = "the `enum_map!` macro expands to `unsafe` code"
+    )
+)]
 pub struct CodeTheme {
     dark_mode: bool,