fix: sanitize custom IM type label before using as IMPP URI scheme

[faraz152]

What

Fixes #499

A contact with a custom IM type containing a space (e.g. "a b") caused sharing/export to fail silently — a 0-byte file was written and an error toast appeared.

Root cause

VcfExporter passes it.label directly as the protocol argument to ez-vcard's Impp(String protocol, String handle) constructor. That constructor builds a java.net.URI using the protocol string as the URI scheme. URI schemes only allow [a-zA-Z][a-zA-Z0-9+-.]*, so a label containing a space throws IllegalArgumentException, which propagates up and aborts the export.

Fix

Before constructing the Impp property, replace any character outside the valid URI-scheme set with a hyphen. Two additional guards:

• Labels that start with a non-letter are prefixed with "x-" (scheme must begin with ALPHA)
• Empty labels fall back to "x-custom"

Testing

Manually verified:

1. Create a contact, set IM field to custom type "a b" with a username.
2. Before fix: Export → 0-byte file, error toast.
3. After fix: Export succeeds, IMPP line written as IMPP:a-b:username.

Compiled and linted against fossDebug variant — no errors or new warnings.

Note on round-trip fidelity

Labels with spaces will be stored as hyphen-separated strings in the exported VCF (e.g. "a b" → "a-b"). Preserving the original label through the URI scheme is not possible without a larger structural change (URI schemes forbid percent-encoding). This is a known limitation documented in the code comment.

[Tchiller]

Hey @faraz152 One small thing in the sanitization chain:

val scheme = it.label
    .replace(Regex("[^a-zA-Z0-9+\\-.]"), "-")
    .let { s -> if (s.firstOrNull()?.isLetter() != true) "x-$s" else s }
    .ifEmpty { "x-custom" }

For an empty it.label, the let block prefixes "x-" first, so by the time .ifEmpty { "x-custom" } runs the string is "x-" (not empty) and the fallback never fires. The PR description says empty labels become "x-custom", but they actually become "x-".

Reordering fixes it, and IMO reads more clearly as a when:

val sanitized = it.label.replace(Regex("[^a-zA-Z0-9+\\-.]"), "-")
val scheme = when {
    sanitized.isEmpty() -> "x-custom"
    !sanitized.first().isLetter() -> "x-$sanitized"
    else -> sanitized
}

Also worth flagging for cross-PR awareness: #439 moves this exact line into CardPropertyAdderMain.kt, so whichever PR lands second will need a rebase that carries this sanitization across.

PS: Little nitty site remark: Regex(...) is constructed per-IM: could be a top-level private val SCHEME_PATTERN = Regex(...)