Fix Shiny app UTF-8 decoding and normalize app-side qs2 reads

[Copilot]

The Shiny app was still rendering existing emoji UI text as mojibake on some environments, and several app flows were still surfacing qs2::qs_read(...) native-encoding warnings. This change fixes the app-side decoding path and routes the remaining serialized model reads through the existing UTF-8 normalization wrapper.

• UTF-8 source loading for app modules

  • Add encoding = "UTF-8" to runtime source() calls in the main app entrypoints.
  • Covers both the current app entrypoints and the legacy heavy UI/server entrypoints so hard-coded UI strings are decoded correctly before Shiny renders them.

• Normalize remaining app-side .qs2 reads

  • Replace direct qs2::qs_read(qs2_path) calls in Shiny model/design/policy modules with the existing read_qs2_utf8() helper.
  • Ensures these code paths suppress the known native-encoding warning and normalize character/factor payloads after deserialization.

• Scope cleanup

  • Revert the unrelated quickstart emoji edits from the earlier pass so this PR is limited to the actual app encoding issue.

# Before
source("modules/load_files_server.R", local = TRUE)
d_obj <- qs2::qs_read(qs2_path)

# After
source("modules/load_files_server.R", local = TRUE, encoding = "UTF-8")
d_obj <- read_qs2_utf8(qs2_path)