fix(ci): use Rust nightly for edition2024 dependency support

Fix for pxfm-0.1.29 parse error:
'feature `edition2024` is required'

**Problem:**
The pxfm dependency (via moxcms → image processing) requires edition2024,
which is NOT stabilized even in Rust 1.83 stable. Error:
'edition2024 is not stabilized in this version of Cargo (1.83.0)'

**Solution:**
Switched all CI jobs from rust:1.83-bookworm to rustlang/rust:nightly.
Nightly builds include unstable features like edition2024.

**Changed:**
- rust-fmt-check: rust:1.83 → rustlang/rust:nightly
- rust-clippy: rust:1.83 → rustlang/rust:nightly
- rust-tests: rust:1.83 → rustlang/rust:nightly

**Why Nightly:**
Edition2024 is still unstable in Rust 1.83 (released Oct 2024).
Some dependencies in the image processing chain require it.
Nightly is the only option until edition2024 stabilizes.

**Note:**
Local development may use stable Rust if dependency versions are locked.
CI uses nightly to handle bleeding-edge dependencies.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.gitea/workflows/test.yml

@@ -10,7 +10,7 @@ jobs:
   rust-fmt-check:
     runs-on: ubuntu-latest
     container:
-      image: rust:1.83-bookworm
+      image: rustlang/rust:nightly
     steps:
       - name: Checkout
         run: |
@@ -47,7 +47,7 @@ jobs:
   rust-clippy:
     runs-on: ubuntu-latest
     container:
-      image: rust:1.83-bookworm
+      image: rustlang/rust:nightly
     steps:
       - name: Checkout
         run: |
@@ -74,7 +74,7 @@ jobs:
   rust-tests:
     runs-on: ubuntu-latest
     container:
-      image: rust:1.83-bookworm
+      image: rustlang/rust:nightly
     steps:
       - name: Checkout
         run: |