Bulk export as HAR ('E' from the history list)

Single-entry export (previous commit) covers "attach this one to a
report"; this covers the other common need - getting a batch of
captured traffic into another tool. HAR 1.2 was chosen specifically for
interop: Chrome/Firefox DevTools, Burp, Postman, and others can all
import it, which a mitmux-specific format couldn't do.

cmd/mitmux/har.go: harEntryFromDetail parses one entry's raw request/
response bytes into HAR's structured fields (method, url, httpVersion,
headers, query string, status, body) via the same net/http parsing the
proxy's own capture path and prettyResponse already use elsewhere in
this codebase - reused, not reimplemented. A binary body is base64-
encoded (HAR's "encoding" field) instead of passed through as a JSON
string: encoding/json replaces invalid UTF-8 with U+FFFD by default,
which would silently corrupt exactly the bodies (images, protobufs,
...) where byte-exactness matters. harDocFrom builds the full document
from a batch of entries, skipping (not failing on) any that fail to
parse - one malformed capture, e.g. a deliberately broken Repeater
request, shouldn't block exporting everything else.

'E' from the history list exports the current view - the visible,
filtered set if a search is active, everything otherwise, same
"respects the active filter" behavior 'x' delete already has and
explicitly the opposite of 'X' clear-all, which always targets
everything regardless of filter. Reuses the same modal path-prompt
state as single-entry export (exportEditing/exportInput), discriminated
by a new exportBulk bool so both share one text-input widget and
key-handling pattern rather than duplicating it. Since the list only
ever holds Summary metadata (no raw bytes), exporting has to fetch each
entry's full detail first - exportHAR does this as a sequence of Get
calls inside a single tea.Cmd, which runs in bubbletea's own command
goroutine, so the UI stays responsive through what's effectively a
blocking round trip per entry. A failed fetch is skipped the same way a
failed parse is; the status line reports the total skipped either way,
not just entries written, so a partial export is visible rather than
silently different from what was expected.

Verified live in tmux against a running daemon: exported 3 real
captured entries (including a binary PNG response) to a HAR file,
validated the output is well-formed JSON with the correct HAR 1.2
structure, and confirmed the base64-encoded image entry decodes back to
byte-identical PNG data (magic bytes checked). Separately applied a
search filter (3 entries -> 2) and confirmed 'E' exported exactly the 2
filtered entries, not all 3 - the same filter-respecting behavior the
status line and help text both claim.

go build/vet/gofmt/test/mod tidy all clean.

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