Handle language switching for every route

doc/2025-08-22-extract-path-params.md

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+# ADR: Implement extractPathParams in CommonHtmxRoute
+
+Date: 2025-08-22
+
+## Context
+
+We use a lightweight HTTP routing based on Go 1.22+ pattern-based ServeMux. Our route paths include placeholders in braces, such as `/en/events/{id}`. The HtmxContext provides a `GetPathParam(key string)` method that uses the request’s `PathValue(key)` to retrieve values for path parameters.
+
+The `CommonHtmxRoute` type already supports building URLs by replacing placeholders via `ToUrl`. However, the reverse operation — extracting path params from the template to build a `[]HtmxPathParam` based on the current request — had an unimplemented stub: `extractPathParams`.
+
+## Decision
+
+Implement `extractPathParams` to:
+- Parse the language-specific path template for placeholders of the form `{name}`.
+- Collect unique placeholder names in order of appearance.
+- For each placeholder name, call `c.GetPathParam(name)` to obtain its value from the current request context.
+- Return the slice of `HtmxPathParam{Key: name, Value: value}`.
+
+This creates a consistent source of truth for path parameters both when generating URLs and when reading them from incoming requests.
+
+## Implementation
+
+- File: `ersteller-lib/htmx_route.go`
+- Function: `CommonHtmxRoute.extractPathParams(c HtmxContext) []HtmxPathParam`
+- Approach: String scanning to find `{` and `}` pairs, trimming and deduplicating keys; for each key, append `HtmxPathParam{Key: key, Value: c.GetPathParam(key)}` to the results.
+- No external dependencies added.
+
+## Alternatives Considered
+
+- Using a regex like `\{([^}]+)\}`. Rejected to avoid introducing a regex dependency and to keep the implementation minimal and allocation-light.
+- Parsing the actual request path rather than the template. Rejected because placeholder names must come from the template, and values come from `GetPathParam`.
+
+## Trade-offs
+
+- The simple scanner assumes non-nested, well-formed placeholders and does not validate balanced braces beyond the first missing `}`. This is acceptable since templates are authored by maintainers and validated elsewhere (routing registration).
+
+## Consequences
+
+- Callers that rely on `extractPathParams` now receive a correctly populated parameter list.
+- No API changes; only internal behavior is completed.
+
+## Notes
+
+- No UI controls were added/changed, so no CSS changes were required.
+- Localization is unaffected since this change does not introduce user-facing strings.
+