Remove Spotless plugin, keep ktfmt via bash scripts

[kaeawc]

Summary

• Removes the Spotless Gradle plugin entirely (version declaration, plugin entry, all configure<SpotlessExtension> blocks, imports)
• Removes the spotless/ directory containing the copyright header template
• Removes the spotless CI job from GitHub Actions and GitLab CI
• ktfmt formatting is preserved: pre-commit hook runs ci/apply_ktfmt.sh, CI runs ci/validate_ktfmt.sh

Test Plan

• Gradle build succeeds without Spotless plugin
• ktfmt CI job continues to validate formatting
• Pre-commit hook still applies ktfmt on commit

🤖 Generated with Claude Code

[github-actions]

Diffuse output:

OLD: app-base-debug.apk (signature: V2)
NEW: app-debug.apk (signature: V2)

          │          compressed          │         uncompressed         
          ├───────────┬───────────┬──────┼───────────┬───────────┬──────
 APK      │ old       │ new       │ diff │ old       │ new       │ diff 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
      dex │  17.8 MiB │  17.8 MiB │  0 B │  60.1 MiB │  60.1 MiB │  0 B 
     arsc │ 468.7 KiB │ 468.7 KiB │  0 B │ 468.6 KiB │ 468.6 KiB │  0 B 
 manifest │   1.9 KiB │   1.9 KiB │  0 B │   6.1 KiB │   6.1 KiB │  0 B 
      res │  56.5 KiB │  56.5 KiB │  0 B │  64.4 KiB │  64.4 KiB │  0 B 
   native │  37.1 KiB │  37.1 KiB │  0 B │  36.5 KiB │  36.5 KiB │  0 B 
    asset │       0 B │       0 B │  0 B │       0 B │       0 B │  0 B 
    other │    46 KiB │    46 KiB │  0 B │ 104.2 KiB │ 104.2 KiB │  0 B 
──────────┼───────────┼───────────┼──────┼───────────┼───────────┼──────
    total │  18.4 MiB │  18.4 MiB │  0 B │  60.8 MiB │  60.8 MiB │  0 B 

         │          raw           │           unique            
         ├────────┬────────┬──────┼────────┬────────┬───────────
 DEX     │ old    │ new    │ diff │ old    │ new    │ diff      
─────────┼────────┼────────┼──────┼────────┼────────┼───────────
   files │     14 │     14 │    0 │        │        │           
 strings │ 241498 │ 241498 │    0 │ 222331 │ 222331 │ 0 (+0 -0) 
   types │  32802 │  32802 │    0 │  30118 │  30118 │ 0 (+0 -0) 
 classes │  28549 │  28549 │    0 │  28549 │  28549 │ 0 (+0 -0) 
 methods │ 146696 │ 146696 │    0 │ 141575 │ 141575 │ 0 (+0 -0) 
  fields │  61016 │  61016 │    0 │  60139 │  60139 │ 0 (+0 -0) 

 ARSC    │ old │ new │ diff 
─────────┼─────┼─────┼──────
 configs │ 106 │ 106 │  0   
 entries │ 372 │ 372 │  0